Mono, the free implementation of MSFT's "it's not Java now!" VM and associated compilers a) works b) is pretty kewl c) gets Linux users on MSFT's bandwagon.
All Mono did for me was get me re-interested in Java. :-/ After all not only is C# only a minor improvement on J2SE 1.4, Mono's GTK# is just functionally equivalent to the (under-hyped) Java-Gnome project.
But here's the kicker: Java is easily available for Fedora Core 3 and Mono isn't. :-(
So no Tomboy, Blam! or Gossip for us FC3 users (one of the most common Linux distros). But I do get RSSOwl - oh, and that's it. :-(