Kurt Schelfthout has just released FsCheck 0.4, a tool similar to QuickCheck but for F#. While working at my internship for Credit Suisse I spent a little bit of time modifying FsCheck to include automatic generators (so you don't have to describe how to generate arbitrary values) and failure shrinking (so the counter-examples are smaller). Both these changes have now been incorporated in to the main FsCheck tool. It is really nice to see the work being contributed back, and that big companies are taking the time to get the necessary legal clearance etc.
I find shrinking to be a particularly potent feature. In one real-world task I struggled to debug a test failure for 8 hours, before shrinking was available. Attacking the same example with FsCheck and shrinking made the reason for the test failure immediately obvious.