1)Four Color To Fantasy
I use it with D&D,
If you need a point buy system for a few level, so that PC get earlier to that prestige class without silly multiclassing and not being overpowered, or...
To produce a Final Fantasy feel in your game, or...
To use something else than magic items to award your PC, by directly improving their skills, or to giv them new talents.
2)Possessors (and all the other products from Philip J. Reed)
It is meant to fill the void left by the disapearance of the mind flayer from the SRD, and it's just perfect for that.
All of the product from PJR are beautifull and readily usable.
3) Tournament Fair and Taverns
If you want to make those tavern games a bit more interesting than a single opposed strentgh check.
Those are the PDF that I use or plan to use (I haven't got the time to try the possessors on my player, but my next gaming session will see its use

)
4)Paladin
It's a full system to play paladin campaign in any kind of setting, too bad that I'm always the GM, I would love to try it as a player.
Plus the cover is wonderfull.
5) A touch of darkness, Gori, Floating City
those PDF aren't pretty, they have a low text density, they do not provide any mechanics, but they provide nice ideas, and they are really cheap ($1!)