Castle is an open source project for .net that aspires to simplify the development of enterprise and web applications. Offering a set of tools (working together or independently) and integration with others open source projects, Castle helps you get more done with less code.
[Borrowed directly from the Castle project homepage]
This is looking interesting. one thing i want to try out is MonoRail:
MonoRail is a MVC framework inspired by ActionPack, a paradigm shift to simplicity.
[Again, borrowed from their homepage]
I have played with the Action Pack, and its interesting, but there seems to be some more interesting stuff in this MonoRail. I am defiantly playing with this over the weekend!