Other
My vote was Other. I'm running a D&D game right now with elements of Omega World, Judge Dredd, and Star Wars. So, those are my highest recommendations, in order.
Otherwise, get Mesopotamia. It is a great adventure AND setting.
I am reading Relics & Rituals Olympus now. So far, it is a good read of incorporating Greek flavor into D&D. I think its predecessor, Excalibur, must be good for arthurian elements, too.
I also like Dragonstar quite a bit, and you should be able to pick it up cheap now that the Essential Collection is released. The Starfarer's Handbook is the best I've read in that line.
Any of the Horizons books is a good read. 64 pages is concise. I liked Spellslinger enough to run it. Grimm is on the list for the free one-shot adventure on the FFG web site. Redline is a neat Mad Max type game. Virtual and Mechamorphosis really push the d20 envelope.
I read the BESM SRD online for free. I think I even downloaded it. I would recommend this as well as reading the d20 Modern SRD online for free. That way, you can try before you buy (or don't buy).
If you want a glimpse of pirate D&D, go look at the free stuff for Skull & Bones on the Green Ronin web site. The book is fun to read, but I opted out of about 90% of the new rules when I ran a pirate D&D game.