There 18k+ open source repos on github for c#: https://github.com/search?l=C%23&q=c&type=Repositori...
There are 44k+ open source repos on github for java: https://github.com/search?l=Java&q=c&type=Repositori...
I'm not even gonna bother for Asp.Net.
Now, for some real advice. 'Open Source' and microsoft don't exactly go together like peanut butter and jelly. Yes, there are some open source projects for the MS ecosystem but it's just not in the culture.
If you don't want to transfer your current skills to the Linux world, you should find a large, open source c# project and just run with it.
Good luck.
There are lots of opportunities for addons/etc, and it's reasonably extensible. Could be a great first project.
It appears that the op may have had a stroke of good luck by picking the perfect OS projects. It would be a bummer to extend yourself in an area that nobody cares about if the whole purpose were to get noticed.
If using Open Source as a self-promotion tool, it's important to choose wisely.
I want to hear about the decision process behind how or why Zaphar chose the open source project he did, and how others here choose their open source projects?
Just a little advice, so that I too (and others reading), can (wisely) choose our first open source project to contribute to.
There is no reason you can't move platforms or languages though. If you know C# or Asp.Net you know enough to get started in ruby, python, perl, or any other language. You don't have to limit yourself in this industry.
However, how could I make any particularly meaningful contribution in a language I am just learning? I feel that any issues or new features in an open source project probably require some fairly advanced knowledge, no?
Blog about what you are learning. Your contribution will be helping others learn from your experiences.
http://paulsantana.wordpress.com/2013/03/30/the-effects-of-a...
You're most valuable contribution will be time and willingness to tackle something.