Horizon

Horizon is a line of five RPGs from Fantasy Flight Games that make use of the D20/Dungeons & Dragons 3.5 edition PHB and DMG to play. In essence, each is a short "campaign gazetteer" covering the bare minimum of fluff and the necessary special rules to play in the setting. Rare and obscure, you can probably torrent 'em if you go looking hard enough.

Grimm

Grimdark RPG where your PCs are kids exploring a seriously fucked up fairytale world. A crazy sort of place where an insane, undead Humpty Dumpty is an evil overlord and Rapunzel went back to her tower after her marriage failed, only for the tower to kill her and now use her hair (still attached to her spider-infested corpse) to lure kids inside it to eat them. Somehow, this got so popular it got spun off into its own RPG game.

Mechamorphosis

It's Transformers with the serial numbers filed off. Do you really need to know any more? The rules are convulated, but the default fluff actually is kind of nifty. Of course, we all know you'd rather run a straight up Transformers D20 game than use it.

Redline

Post-apocalyptic setting that basically revolves around using vehicles to travel at high speed through the wasteland without getting killed. Sort of Fallout meets Mad Max.

Spellslinger

Low fantasy setting mixed with the Wild Western. No 9 level spellcasters here; you get a select few spell-like abilities via the Brands trees, and you can't multiclass 'em. On the awesome side, it includes pretty decent D20 rules for Western-grade guns and the fluff is actually pretty cool. Oh, and it's got a race of carnivorous killer horse-things as new mounts. On the downside, there's only one new race, and that's anthro wolf totally-not-American-Indians.

Virtual

Ever heard of a Disney movie called "Tron"? Ever wanted to RP life as one of the programs inside? Well, this game will fill that craving.