There is a decent survey of the retro-clones here. I should note, however, one mistake: Basic Fantasy is not a retro-clone of the D&D Rules Cyclopedia (indeed, Basic Fantasy isn’t really a ‘retro-clone’ at all, as it does not try to ‘clone’ directly an earlier version of D&D, but rather is a ‘new’ game inspired most directly by Basic D&D, but with some AD&D and d20 elements as well).
Also, James Maliszewski of Grognardia fame offers a history of the retro-clone 'movement' here. I was especially amused by the following observation: “If the retro-clone creators are the "engineers" of the movement, the bloggers are its ‘philosophers.’”
Heh. It seems that I cannot escape from being a philosopher, even in my procrastination endeavours! (Although I have to say that this blog has not been especially ‘philosophical’ in its treatment of old school RPGs – I’ve been focused primarily on house rules and presenting my campaign setting. This may change, though, as time goes on.)
(Hat tip to Bat in the Attic for the links to the above articles.)