For my fourth year project, I somehow managed to convince four other people that they wanted to help me create a 2D/3D physics engine.
Leveraging everything I had learnt from my third year project, it was surprisingly successful. It has already been used in two games, with possibly more to come.
Unfortunately, due to some bad design decisions it wasn't usable as a library for a game, but the lessons I learnt from it were invaluable. It was a fun project and an excellent prototype for what became Large Polygon Collider.
You can download my source code from my Subversion repository at: http://svn.draknek.org/physics/code/.
Physics and Collision Detection - talk for Warwick Game Design (20/06/2008)