I tend to be a bit skeptical here it may be well written, but with a small community help can be hard to come by, and it's probably not a well tested and stable engine, compared to the others. I don't know much about it, and it hasn't really gained a substantial community. TrueAxis - A recent appearance on the scene.It's got heavy Sony backing, it's nicely cross platform, and it's developed by people who know what they're doing. This is the newbie of the scene, and it is actually open source. Bullet - This one's lead by a former Havok employee, who is now employed by Sony (although I can't remember if that's SOE or SCEA).
It is open source and wildly fast, though. Just pull up their box stacking and you can see massive jitter. This one is awful even their demos don't run stably. It is a well designed engine and it works well, but it's still one of the slower ones out there. It's spent the last few years trying to gain performance without sacrificing the other two.
That said, the free systems I'm aware of are: Building a fast, accurate, and stable solver is extremely tricky, and people like Havok are understandably protective of the technology.