I did a post a couple of days ago on SharpDevelop, an alternitive IDE for c# development on Windows. Alan responded to the post by mentioning Mono Develop, an IDE for .NET Linux development using the GNOME tool kit.
Sounded like a greate idea so here's a link to the Mono Develop site. I was able to use Mono Develop version 1.0 while at Vanderbilt. It was a bit twitchy, but work well for a version 1.0 release. I hear that the 2.2 version works well and is usable on Linux, Mac and Windows. I plan to try a project using Mono Develop.