The Spellcraft & Swordplay Basic Rules (available at DriveThruRPG and RPGNow) are free and complete for levels 1-3.
The Tomb of Sigyfel for Labyrinth Lord.Sue used this when she first tried her hand at running a game. I was amazed how she took such a simple "quick start" module and made it into one of my favorite gaming memories today.
Depending on your definition, Castles & Crusades quick start rules are available here: http://www.trolllord.com/cnc/ccqs.html
The brazilian OSR game, Old Dragon, have a free "Fast Play" version, with rules up to level 5.http://olddragon.redboxeditora.com.br/fast_play/