Ximian's open source implementation of Microsoft's .NET platform and C# language. Ximian is now owned by Novell.
Mono (Spanish for "monkey") is a highly ambitious project, with a lot of work done by just one person (Miguel de Icaza, a GNOME legend) - the fact that they have gone so far in such a short time is very remarkable!
Mono includes a virtual machine for Common Language Infrastructure (CLI), a class library, and a C# compiler.
The virtual machine includes everything needed to run the CLI bytecodes, including a class loader, JIT compiler (works fine in Linux), and garbage collector.
The C# compiler works already pretty well. It is remarkable for two things: First, it's written in C# (being thus first of its kind, and an interesting academic experiment...), and it's already able to compile itself. This means it's going to Rule.
The class library is not yet that complete, however, and apparently not yet complete enough for anything totally serious. Particularly missing is the Windows.Forms class hierarchy (well, that's not technically part of the language specification, either, but it's still vital, as anyone can imagine), but there already is a wrapper for GTK+ called Gtk# that makes it possible to write GUI apps in C# that run under Linux (but not yet under Windows).
All in all, the project is very interesting, even when the platform itself is not that interesting to the open source folks in general. Yet again, it seems, Microsoft has "let go" an open technology that is likely to go out of their hands if the open source world is doing their job well enough!
Home page: http://www.go-mono.com/