Coding .NET for Linux Environments (free software)

Grasshopper 2.0
Implements a cross-compiler that translates M$ Intermediate language to Java bytecode
It seems the free edition only supports Tomcat though, so for commercial App Servers like IBM Websphere, you'd have to buy the enterprise edition.
product overviews and reviews

Mono
"Mono allows your existing binaries to run on Linux with copy-deployment."
It's basically an open-source .NET framework for Linux

IDE:
MonoDevelop
"free GNOME IDE primarily designed for C# and other .NET languages"