|The cover to OSRIC, one of my favorite retro-clones|
Retro-clones are not perfect copies. Due to the OGL and copyright laws they cannot include everything. For example, beholders are off limits as they are owned by Wizards of the Coast. Also, retro-clone authors often clean up the rules and add their own interpretations in order to make the games easier to understand and play. The grappling rules in 1st edition AD&D include a page of charts, while in the retro-clone OSRIC they are reduced to a paragraph.
What do I like about retro-clones? Many of them are available for free. They are easy to play and fun. Did you just find a bunch of 1st edition modules at a garage sale but you don't have your core books anymore? Download a retro-clone and have fun. Never played original D&D, where all weapons did 1d6 damage? Download the Swords & Wizardry WhiteBox Rules and have fun!
Here is a list of some retro-clones with a brief explanation for each:
Basic Fantasy - This is a clone of Basic D&D, except it uses ascending armor class and race and class are separate. If you know someone who has never played a fantasy rpg before, I would give them a copy of this.
OSRIC - OSRIC is a clone of 1st edition AD&D. Use this if you have a bunch of AD&D material or you were a fan of the system. Doesn't have the monk, bard, or psionics, but otherwise is very complete and full of options. It is also available on rpgnow.
Swords & Wizardry - a clone of original D&D. Comes in three flavors, the white box version which emulates the original booklets (0 ed), one which emulates the original boxed set and expansions (1st ed), and the complete version which is more like AD&D lite. Includes rules for descending and ascending armor class. Play this version if you want quick and dirty fun with lots of house rules.
For Gold & Glory - a clone of 2nd edition AD&D. I plan on using this with the Dark Sun boxed set that I just picked up at the used book store. Also available on rpgnow.