Damn, I'm having a tough time choosing between two, mostly because they are so similar.
Basic Fantasy and Swords & Wizardry. Both solid retro-clones, both well supported, both well loved by their creators. Basic Fantasy is a tweaked version of Basic Edition D&D, with modifications like an ascending Armor Class. S&W is virtually the same thing, only with descending Armor Class as the norm (with ascending available in brackets on the table).
Honestly, good luck on telling them apart. I tend to think that S&W is the more polished product in terms of appearance, but even that is just a nudge.
There are a lot of other options if you were to ask the question in terms of "pay what you want," like FATE, for example. But since I'd be likely to pay at least something, it wouldn't be free.