The Basic Fantasy Role-Playing Game http://www.basicfantasy.org/downloads.html -This game has only recently been brought to my attention. It is unique in that it uses the 3.5 d20 system, is very easy to understand (even for 2nd and 3rd graders), and that it tries to use the flavor of D&D 1st edition. I'm very excited about this game in particular.
Swords & Wizardry Core Rules and Whitebox Edition http://www.swordsandwizardry.com/?page_id=18 -The White Box is a clone of the original D&D White Box or 0 Edition; the first ever D&D game. The Core Rules are a clone of the D&D Red Box which is still 0E but with more rules.
OSRIC http://www.knights-n-knaves.com/osric/ -A clone of the D&D 1st edition rules, much easier to understand than the actual TSR rulebooks in my opinion, but without some of the cool old art.