URPG
Unknown RPG is an OSR* with some elements from almost every edition of TSR D&D stripped down and rebuilt with new ideas added in, it's fairly compatible with most TSR dungeon modules, though it tends toward balance to use with 1e modules. It's made as an online document to be updated quickly and easily. Please feel free to make copies and make your own changes directly to your copies.
BURPG is a very basic version meant for new players that goes up to level 5, I've found it considerably better and faster to get started with than even Holmes Basic. Character creation can be walked through with the character sheets in a couple minutes. This version has been extensively tested.
AURPG is an advanced version with many more experimental rules and options that goes to level 11. There has been some testing, but it's still a work in progress.
Basic Unknown RPG Player's Guide
Advanced Unknown RPG Player's Guide
Unknown RPG Random Character Tables
*Old School Renaissance Dungeons and Dragons Tabletop or Pen and Paper Role Playing Game.