(Note: The original website, including a series of tutorials and all API documentation, has been archived here.)
OPAL (Open Physics Abstraction Layer) is a C++ software library which provides a common API for any number of real-time physics engines (collision detection + rigid body dynamics simulation). That was the original goal, anyway. Eventually, it became clear that the real value of OPAL was as a high-level, object-oriented wrapper for ODE. It features a very simple API, smart default parameters, a handful of intuitive objects (e.g., Solids, Joints, Motors, Sensors), and XML-based file storage for complex objects.
OPAL has been used in a few dozen software development projects, mainly in independent video games, grad school class projects, and funded research projects in and around the VRAC. I also used it for my master's thesis experiments on reinforcement-based motor learning.