Can I compile .NET C# code on my Mac? I need a way to cross compile a short piece of .NET C# code to a dll, which will then run a client's Windows environment.

I run OS X 10.9.2 on an iMac.

Does anyone know what tools allow this?

I would rather not install Parallels and Windows if I can find a way to cross compile this without needing that heavy of an installation of Microsoft tools.
Check out the Mono Project. It may be what you are looking for.

http://www.mono-project.com/Mono:OSX

MonoDevelop is a cross-platform IDE primarily designed for C# and other .NET languages. MonoDevelop enables developers to quickly write desktop and ASP.NET Web applications on Linux, Windows and Mac OSX. MonoDevelop makes it easy for developers to port .NET applications created with Visual Studio to Linux and Mac OSX maintaining a single code base for all platforms.