The Mono team has released their better version so far, including a number of huge improvements such us .NET 2.0 compatibility (yes, generics and everything), the initial Moonlight (Silverlight for Linux) and many more.
But the best is yet to come: they already have LINQ and lambda expressions (.NET 3.5) almost there, ParallelFX (the multi-core .NET library) and totally new stuff.
When people talk about Mono, they normally focus on its compatibility with the .NET platform and how good it is for a number of companies tied to the Microsoft stack. But let me look at it from a different perspective.
[I like their new C#shell, and the fact that it works ok with firebird 2.1 on all platforms:macosx,linux,windows ]