As very close cousin of Java, I can't help but like C#. I find it most interesting to see the differing approaches that Microsoft and Sun/Oracle have take with these languages. I started working on .Net when the platform was first released and since then I have dabbled around in most of the languages on the .Net platform.
My most recent work in C# was on a legacy system at Pearson VUE. I also designed an Election Tracking system for Wisconsin Public Radio that was written in C#. That was quite a while ago now (it is amazing how time flies!). It was part of an open source project with UWEC. You can have a look at the code at electiontracker.googlecode.com
I often say "If you look at the code you wrote last year, and you're not horrified by it, you haven't learned anything in the past year." Well, I'm pretty horrified by this code so I guess I have learned a thing or two since then.