Rulebooks:
Adventures:
Some good cheap games found on Amazon:
Modern Adventuring & Plunder - a good solid system
Heroes & Monsters - a Black Hack clone
Sojourn - this one is a little over $10, but is quite nice. Note that there is a cheaper pocket-sized edition of this game available, but people have said its very hard to read, so I would go with the full-sized version
Rules sets:
Adventures:
Big book of monsters:
Sure!
That's everything I was able to find so far, they are mostly retroclones and all of them are broadly OSR games, I already bought BFRPG and I'm strongly considering buying most of them!