After the success of b2BuoyancyController, I'm releasing a pack of useful controllers to do other common tasks. I hope you appreciate this, these were boring to write. Demo

Following the poll, buoyancy seemed to be the popular choice for Box2D feature. I decided against the fancy and probably unreliable technique I was going to use, and have produced a "controller" based on good old, geometry, cribbing a fair amount of Erin Catto's notes on the subject. Demo

Box2D has always had a function called TestSegment for colliding segments with shapes, but it really needed conversion into proper raycasts. Check out a Demo.

I've implemented this in Flash and C++. It's the real deal: it uses Box2D's internal sweep and prune structure for efficient calculations. You should see the best performance gains on the previous naive solutions when doing very long diagonal casts, or when doing a long cast that hits something immediately.