1. Basic Fantasy is a very nice, complete system. The free core rules are all you really need, but the many free supplements let you create the game you want.
2. I haven't played it yet, but Venger's Crimson Dragon Slayer D20 looks like it would be fun. "D&D on Arcade Mode" seems like a good selling point for this one.
3. While Ben Milton's Knave RPG is not free, there are several hacks of it that are. I created my own Knave hack, titled
Book & Blade. It's a 7 page rules light system for running OSR adventures. Please feel free to take a look.