Like Simon says
Mono[
^] is your best bet.
However the current Mono version does not support .NET 4.0. You will have to wait for Mono 2.12 which is
planned[
^] for the fall of 2011.
So the question is are you able to target .NET 3.5, in which case you can make use of Mono right now.
See this MSDN
article[
^] on how to change the targeted .NET Framework.