jmucchiello
Hero
This is an important point. The original clones (original. clone. right.) were just attempts to create OGL legal SRDs for old editions of D&D. (4c is a retro-clone of the MSH FASERIP system. But I don't think you want to include it on this list.) Since then retro-clone has kind of subsumed any "old school" style game.You'll need to decide what constitutes "clone" versus what constitutes "inspired by."
Take, for example, Goodman Games' upcoming DCCRPG. It is not a clone (copy) by any stretch of the imagination. But it is compatible with most old D&D modules and most of the retro-clones as well.