CollidableModel is a fully integrated collision detection system for XNA. It consists of a content processor that can add collision data to arbitray models during the build stage. A set of runtime classes perform highly efficient mesh - sphere collision detection based on the pre-computed data. For average sized models (10k - 100k), the intersecting triangles can be usually determined in less than 1 ms.
The system is based on my previous samples, using a Binary Space Partitioning (BSP) tree as data representation. Note that it only works with static (not animated) models.