Jitter Physics
Via Corporal X comes the word (well, sort of: I heard of this thing before, just were too lazy to actually post it here – looking at the news stream tells me that I am not alone on the slacker front, though) about a new, shiny and managed only physics engine named Jitter Physics. Says their PR:
Jitter is a fast and lightweight managed engine written entirely in C#. It includes it's own math framework and does not rely on third party software. The engine was written by the author of JigLibX. Jitter has some additional features including an interface following the .NET coding guidelines.
It is currently in beta and word is that “it is about 100% faster than JibLibX but also much more stable when stacking objects. It supports more default shapes (example: cone, cylinder) and it's code is just nice 
What is missing at the moment is vehicle and terrain support”.