I was never really that into the Microsoft world but others have mentioned places to look for OpenSource projects for those platforms.
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.
That is true. Funny you mention Ruby as I have just completed Code School's Try Ruby course this weekend!
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?
A lot of my blogging at that time was about what I was learning. In fact those posts were the most instrumental in getting me the job in Chicago. My blog showed up for a lot of howto searches for perl. Which got me noticed by a company needing perl programmers.
Blog about what you are learning. Your contribution will be helping others learn from your experiences.
You'd be surprised what you can tackle even when learning a new language. A lot of the bugs in an open source project aren't unfixed because they are tricky but because no has had time to work on them.
You're most valuable contribution will be time and willingness to tackle something.
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.