I've been searching on Google for some C# programs. I have found Paint.NET. Is that the only C# program out there? What about games? Someone have a link to 3D game in C#?
If you want to write games for Windows or the XBox360 you can use XNA... also their are tons of apps all over the place that use C#. You can also find projects that were written in .Net on CodePlex
Quake was ported to C#/Silverlight: http://www.innoveware.com/quakelight.html
Play it now: http://www.innoveware.com/ql3/QuakeLight.html
SourceForge might be a good place to start. On the projects page select C# as the programming language and search. There are currently about 17,000 C# projects.
Browsing the the source for a large project may be a bit daunting if you are learning - I'd recommended trying to find something simple to start with.
Edit: Another couple of quick links to games
- Sudoku in C#
- Gem Towers (much more ambitious 3D C# game)
You could have a look at Unity3D - it uses Mono to let games run on Mac and PC. All behavioral code is written in C#. There are games by both EA and Cartoon Network as well as loads of corporate and indie games based on Unity. Unity 3D Game List
A very good resource is the XNA Creators Club.
This has some great resources for learning (the starter kits for XNA are full games, with artwork, written in C#). In addition, there is a community driven site full of examples of 2D and 3D games written in C#.
You can have a look at SharpDevelop and/or MonoDevelop, two spen source .NET managed IDE.
Also, as others stated, there are plenty C# OSS frameworks that you can study. Here is a first list.
Here are some 3D engines and whatnot created in c#:
http://www.multiverse.net/index.html (Reference to C#)
http://axiomengine.sourceforge.net/wiki/index.php/Main%5FPage
http://www.visual3d.net/