Mono is an effort to create an open source implementation of the .NET Development Framework.
Resources:
- The MonoDevelop IDE (there's also SharpDevelop)
- Some apps: Dashboard, Blam (an RSS aggregator), Muine (a music player).
- Mono on the Mac, official page
- Bundles for Standalone App Deployment
Language Ports:
- IronPHP - the beginnings of a PHP port to the CIL
- IronPython - an interesting (if still somewhat limited and less than open) port of Python.
- Boo, a Python-inspired language