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Sending a data from android phone to pc?

I am creating a "tracking system" for my android phone. Basically, the phone would transmit it latitude/longitude to a C# program that will receive the coordinates and display it on a map. I managed to get the phone's latitude/longitude coordinates but I need a way to transmit that data to my C# application running in my PC. I know my way around C# and Java but never really got into network programming. Is there a way to do this ?

I'd look into C# Webservices, very powerful. The communication protocol is SOAP (Simple Object Access Protocol) which is a popular and well supported standard.
Good starting reference: Your first C# Web Service

I can't speak to the C# side of it, but if you write a C# server to receive post requests, the Apache HttpClient makes it relatively simple to send requests. There are a bunch of other questions on SO that will help with setting up the client side on your phone if you poke around a bit.

You have different way to do it.
The first is to send data over the cloud to interact through distant server. So the both devices have to be connected to Internet
The second id to send the data over a local network so the data have to be on the same network and have to share their IPs
The third is to use adb forward which allows to build a socket connection between the 2 devices over a USB cable. The requirements are that the phone have to be on debug mode I think and that the driver are installed on the PC so the phone is recognized by ADB.
I can add one more by using a Bluetooth connection. Find information here

The correct way to do it would be to build WCF service which allows many different protocols to receive data. This would could be a very powerful solution that can be extended in future.
The fastest and easiest IMO would be to create ASP.NET MVC application, define a POCO Model for your longitude/latitude and create an action that would take your Model as parameter. Then use HttpClient and HttpPost on android to send the data. The trick here is to name the variables in your post request same as ones you define in your C# Model.

Programming P2P application

I am writing a custom p2p program that runs on port 4900. In some cases when the person is behind a router, this port is not accessible from the internet.
Is there an automatic way of enabling the access to the port from the internet. I am not really sure of how other p2p applications work.
Can anyone please throw some light on this?

P2P connectivity in a nutshell. Assume we're talking about UDP here. The steps below can also be applied to TCP with some adjustments.
Enumerate all your local IP addresses (usually only 1). Create a UDP socket on a given port number** for each adapter with an IP address.
For each socket created in step 1, contact a STUN or TURN server with that same socket to discover your external IP address and to discover what the internal port number maps to outside of the NAT (it's not always the same port value). That is, your local address 192.168.1.2:4900 might be 128.11.12.13:8888 to the outside world. And some NATs don't always use the same port mapping when using the same local port to other IP addresses. TURN will also provide you a "relay address". You can also use UPNP to get a port mapped address directly from your router, if it supports that protocol.
Through a rendezvous service (SIP, XMPP, instant message, web service, email, cups with strings), publish your address candidate list to a service or send a notification to the other client that says, "hey, I want to connect with you". This message includes all the "address candidates" (ip and port pairs) collected in steps 1 and 2.
The remote client, upon receiving the invite to connect, performs step 1 and 2 above as well. Then sends back his candidate list through the same channel that he received the inviter's candidate list on.
Hole punching step. Both clients, start sending test messages over UDP to the other side's address candidates and listening for the same messages on their end. Whenever a messages is received, reply back to the address from which it came. Eventually, the clients will discover that they have a pair of addresses that they can reliably send datagrams too. Typically, one endpoint makes the final decision on which address pair (sockets) to communicate with and the protocol facilitates this endpoint telling the other endpoint this decision.
**- usually best to not to rely on a well known port for P2P clients. Because two clients behind the same NAT or firewall would not likely be able to use your software at the same time.
Here is a quick summary of some technologies to explore.
STUN - Is a simple server and protocol for clients behind a NAT/route to discover what their external IP and port mappings are.
TURN is an expansion to STUN, but supports relaying for P2P connectivity scenarios where firewalls and NATs prevent direct connections.
ICE is a set of steps by which STUN and TURN are used for setting up a P2P connection. ICE is a formal protocol for steps 1-5 above. Two excellent set of slides on ICE are here and here.
WebRTC is a variant of the ICE standard as well as a reference library for make P2P sessions with STUN and TURN.
UPNP + Internet Gateway Device Protocol - Some routers support this for hosts to automatically obtain port mappings.
libnice is an open source C library for Linux (and might work on windows) that implements ICE.
libjingle is another ICE implementation (in C++) from Google. For Windows and Linux.
PJNATH is a library within the PJSIP suite of coding libraries. It is a good implementation of an ICE stack (C code) and has been ported to a lot of platforms. (Windows, Linux, Mac, iOS, Symbian, and soon Android).
And finally, I have a blatant plug for you to use my STUN server code base.

There are solutions in some cases, see UPnP: https://en.wikipedia.org/wiki/Universal_Plug_and_Play#NAT_traversal
My home router allows this, basically, the NAT can be configured automatically by the proper request from the computer.
I would not count on this to provide a big improvement in your availability, because not that many routers both support that and have it enabled.
EDIT: #David suggested this SO question for a .NET library for UPnP: Is there a UPnP Library for .NET (C# or VB.NET)?

I would use WebRTC technology as an open source framework for such application.
Official Website
In fact it is an open source project which supports all necessary for peer-to-peer technologies out of the box:
ICE and STUN (NAT traversal)
DTLS and SRTP (security)
AVPF for quality of streaming.

This may be a little more complicated than what you're looking for, but TCP Hole Punching is a technique that should work. http://en.wikipedia.org/wiki/TCP_hole_punching
Alternatively, UPnP works great for routers / firewalls that support it.

You have another option that is NAT Port Mapping Protocol (NAT-PMP)
NAT-PMP is widely used by VoIP applications like Skype or BitTorrent P2P clients.

For the simple beginnig I would recommend to see hole punching technique. Great video here. But be sure it will not always work relating the network topology. This solves the ICE technique which discovers how connection could be established.

Idendifying caller number. What is the easiest way?

I have PC and VOIP phone. When someone calls to my account and I answer it using VOIP phone, I want the phone number to be stored on my PC.
What is the easiest way to receive caller number? Is finding a VOIP Java/C# SDK and developing my application (just to get a caller number) the only way or are there some clever shortcuts?
Note: I am familiar that softphones are displaying the callerID but there is no (reasonble, non hack'ish) way I can read it from softphone.

Depending on your softphone you might be able to configure it to save a calls log. Alternatively you could obtain CDRs (call detail records) from your VOIP server.
Another route to go would be to monitor incoming VOIP TCP connections and extract caller data from sniffed packets.
Or you could write a small program that will run in background and check periodically which is the active window. If the active window is your softphone, then it will take a screenshot and dump it to a folder.

What Vlad said. Providing the hardphone model details will get you more details :-)
If the PC is sure to be on all the time you want to operate the phone, you can install a simple SIP proxy on the PC that does nothing but logging.
If a softphone is an option, many of them can be scripted, I love twinkle for example.

Using wireshark (or more to the point tshark, the command-line version) you can build a capture spec that will only capture incoming SIP packets on UDP port 5060, and then only if the SIP msg is INVITE. Then you just have to look at the "From" line to see who called you (if it's filled in; it should at least have a number).

How can two C# apps send messages over a WiFi Network?

Lets say I have my C# app installed on 2 laptops connected to a WiFi Wireless Local Area Network.
How can these apps send messages to each other? What method or library can I use? I heard of using sockets but I have no idea how to work with these.

You could use WCF to build a communication pipe between the 2 applications. WCF encapsulates the sockets into a more manageable interface. You can start here.

Basically, you'll want to do it the same way you would in any other language. You'll open a network connection of one flavor or another (raw TCP or UDP, or a higher level protocol like HTTP) with one side acting as a server and the other acting as a client. Then each side can write data through or read data sent by the other side. It's can get pretty complicated from there. If you Google "C# Sockets" or "C# HTTP", etc, you'll find quite a few tutorials on the subject.

This is a very good article on sending C# objects (which could include whatever messages that you want to send) over a Socket connection using the Binary Formatter. Although it is not the most efficient, it is quite easy to grasp and get working.

Share data in LAN

I'm building a music library program, and I want to have the ability to share the library in the LAN. How can I discover others who share their library? I'd like to find others' libraries without typing in IPs and stuff.

Apple uses mdns (they call it Bonjour) to broadcast and discover music shares on the local network without any user configuration.
Many other manufacturers implement UPnP/AV for the same.

Probably the easiest way is to use UDP to periodically send a message (containing some info advertising your library's presence) to the broadcast address. This will be received by all the hosts on your subnet (and perhaps further, depending on your router configuration). If your app listens for these messages from other hosts, it will over time be able to find all the other instances of your app on the subnet.
Edit: found this question which has answers that go into more specifics of what I'm talking about.

SNMP
This protocol was designed for what you are looking to do.
There are several libraries that you could use that implement SNMP which would make it easy to send and receive.

You could get your application to send the fact that the library is shared to a central server - in fact just send the location.
Other instances of your application could then just ask the server for the list of shared libraries.
The advantage of this is that your application isn't constantly broadcasting.
The disadvantage is that you need a central server.