Well, somebody asked the question on EN World: Retro-clones - explain?
And Philotomy Jurament has a very nice answer :
- Swords & Wizardy (White Box) – OD&D74 without Supplements. Should be very close.
- Swords & Wizardry (Core) – OD&D74 + Supplements. Mostly compatible with TSR D&D, in general.
- OSRIC – 1e AD&D. Very close.
- Labyrinth Lord – Classic D&D (similar to B/X). Very close.
- BFRPG – not really a clone. Mostly like Classic, but more of a mix (separate class/race, d20-style AC, etc)
- GORE – BRP (Runequest/Call of Cthulhu). Very close.
- Mutant Future – similar to Gamma World, but not really a clone (awesome game, though)
- ZeFRS – Conan
- C&C – not really a clone, more like a via media between TSR D&D and WotC D&D, favoring TSR D&D, IMO. Or like the “AD&D 3rd Edition” that never was.
I’ve played LL before (→ 2008-04-20 Palace of the Silver Princess) and decided at the time that I didn’t like it because of the high attrition rate. My hope is that the next time I run an old-school one-shot I’ll be using one of the adventures that actually made it into the various best-of lists out there.   
When I started my current gaming in late 2006 I used GreyWulf’s M20 rules – they are basically D&D 3E (very!) light rules, not a true retro game. Then again, at least one person has rewritten these rules to be more old-school resulting in M74.
Tags: RPG publishing Old School