I found MonoRail after following a link buried in the comments of Zac Bowling's Hot or Not post from a couple of days ago. Now I'm wondering how I managed to go so long without hearing about it before...
Obligatory what the heck is he talking about quote from the Castle website:
MonoRail (former Castle on Rails) is an attempt to provide a port of Action Pack for .Net. The Action Pack way of development is extremelly productive, very intuitive and easily testable.
There's also a few Flash Demos you can watch. Definitely worth trying out.