Novell's Mono 2.0, an implementation of the .NET framework on Linux, has just been released. According to the Mono project leader Miguel de Icaza, the latest release supports most used .NET features, thus leaving some features including WPF unsupported. Download is available at the Mono site.
On a releated note, Moonlight 0.8, an open-source implementation of the Microsoft Silverlight runtime (based on but independent of the .NET framework), is available for download at the Moonlight site. Moonlihght is a browser e.g. Firefox plug-in primarily for Linux and other Unix/X11 based operating systems.