Here are some of my favorites at the moment. Some new, some older. I talk about them when I get the chance.
Omega World d20 mini-game from the Polyhedron side of Dungeon # 94. EDIT: THIS IS THE BEST ONE ON MY LIST, SINCE THAT'S WHAT THE POST REQUESTED. Author is Jonathon Tweet, which probably explains its brillaince. Its about 40 pages but does what many, many other d20 settings or games struggle of fail to do in hundreds of pages. It really captures the feel of the classic Gamma World game (for me, at least) without an overwhelming number of new rules. Plus, it has a bestiary and tools for the DM to run the game. A gem.
EDIT: FOLLOWING ARE HONORABLE MENTIONS.
Judge Dredd d20. I really like this game. I have all the books, even though I use only the core book and the modules (imagine a setting with adventures!). I think the adaptation of the 3.0 rules to the JD future/sci-fi/post-apoc genre is very well done. Again, it is without too many new rules that could make the game unnecessarily complicated; and they give the reasons for the design choices they made. My only complaint is the intro adventure was apparently cut from the book.
DragonStar d20 Starfarer's Handbook. The rest of the line was a little lackluster for me, but the first book--wow. Another great adaptation of the 3.0 D&D game to a future setting, and it beat Star Wars d20 to market. I may never play it again, but it still is good read.
Ancient Kingdoms: Mesopotamia. The presents a mini-campaign setting with a good-sixed module in one. The rules for bronze-age equipment are short & sweet, as are those for new human sub-races and new deities. I wouldn't use the rest of the variant rules, but he module looks like a lot of classic fun in the style of Robert E. Howard.
The Last Hero in Scandinavia. The expereince chip rules are great for a more heroic game. The setting information is just a little blurb oin how stylized vikings act and what d20 classes probably work best in the advneture. I would love to run this module as all-human with some barbarian, cleric, fighter & rogue northmen and one arab bard as an homage to The 13th Warrior.
5 is enough.