I've wondered this a bunch of times. The answer that comes to mind is: there's no point since VS is so feature rich, but on the flip side, this the fact that something has already been done hasn't stopped people in the past.
A:
There was a C# plug-in for Eclipse a long time ago, but it wasn't really production quality by a long shot. I haven't seen anything else coming into the Eclipse ecosystem to support .NET.
Jon Skeet
2010-09-29 16:06:36
A:
Emonic is a more recent plugin for C# on Eclipse. It's meant to work with either Mono or MS.NET.
Sean McMillan
2010-09-29 16:27:26
A:
While not a whole C# environment, it's probably worth mentioning the Silverlight tools for Eclipse:
http://team.silverlight.net/announcement/eclipse-tools-for-silverlight-now-available/
nos
2010-09-29 16:41:46