If you're a reader of this blog than you probably know about this already.  But who really knows and just in case you don't, here's a link to Duncan Mackenzie's great page of C# tools.  You'll find links to all sorts of .Net libraries, compilers for your platform of choice (definitely try Mono if you're on a Mac), obfuscators, decompilers, profilers and much much more.  My favorite tool on that page?  Definitely Lutz Roeder's Reflector.  One of the few tools that I feel is "just right" and an indespensible tool for any .Net developer.  It was on this page that I found the C# mode for emacs.  I use it mainly for font-lock, but it's still really useful.  If you haven't checked this out you should, you just might find something useful for you now or in the future!