It's been about 10 years since I used C#, but even in it's nascent form I enjoyed programming in it. Also dev studio for C# is the only IDE that I've ever enjo

It's been about 10 years since I used C#, but even in it's nascent form I enjoyed programming in it. Also dev studio for C# is the only IDE that I've ever enjoyed using (for C++, not so much ;-) ). But the OP did specify for non-Window OSes and for Open Source. And I don't think they meant that the language itself had to prove itself, I think they meant that it needs to prove that it can inspire programmers in those other cultures. That's quite a reasonable statement IMHO. MS stumbled out of the gate with this. They made a reference implementation, but used their crazy shared source license for it. The Mono/Gnome guys had a huge love affair with MS (to the point of trying to make their own implementations of Com objects and the registry (!)). MS was vague about their feeling about Mono for far too long. Even now, my understanding is that Mono is very careful not to tread on any toes when re-implementing things that are in .Net. And I'm probably one of the few people who used the open source .Net library (whose name I forget now... Mobile.Net... or something) and was constantly cursing that MS made it difficult for code to run on both that platform and a full blown windows .Net platform. If you want to write proprietary code on Windows? Absolutely C# and .Net all the way (or, these days F# is looking very nice). But open source on a Posix box? Not touching it with a ten foot pole. I've got lots of better options.