Replying to bump the thread. Looks like FreeSOLID is basically an old version of Gino van den Bergen's SOLID code, which implements the well known GJK collision detection algorithm. I think its well used. I expect the newer, non-free versions are faster and more robust. But, you may find FreeSOLID to be fine. You may also want to look at Pierre Terdiman's OPCODE and/or the various things available from the University of North Carolina at Chapel Hill (though be warned that University code ain't always the prettiest):

OPCODE
UNC Collision Detection / Proximity Query Packages
SOLID