ilasm.exe (MSIL assembler) and ildasm.exe (MSIL disassembler) that come with the MS .Net Framework SDK are all you need. For debugging, use DbgCLR.exe that comes with Visual Studio .Net.

I have cracked/keygenned quite a few .Net apps now, but it has become somewhat boring because it is so easy. There is not much of a challenge in that area. Unpacking is definately much more fun!

What .Net program is it that you are trying to crack? If it is one that I have already done, I may write a tutorial on it if I have the time.