Dark Sun stuff I liked:
Thri-kreen of Athas - very indepth description of the kreen, with a heavy focus on the two subraces populating the Tablelands but a bit on the huge Kreen empire to the northwest (beyond the Jagged Cliffs).
Defilers & Preservers - perhaps not all that interesting if you're going to be using DS with 3e, but I did like it back in 2e. It was basically the Dark Sun version of the Complete Wizard's Handbook, only better.
Earth Air Fire & Water - the priest book for Dark Sun. Real nice, but I think they made the elemental clerics a bit too goody-two-shoes and had to insert the paraelemental clerics to compensate. I would have preferred a more balanced view of the elemental ones. Also a bit of info on druids and templars.
Freedom! - Good starting adventure (if you use the original box), though it does change the setting a bit.
The Will & The Way - The DS version of the Complete Psionicist's Handbook, or what the CPsiHB would have been if it hadn't had to introduce the whole concept in the first place. Lots of neat stuff for psionics, including how they're viewed in different places and a whole lot of new powers (though some are "a bit" overpowered, like Kinetic Control).
Monster books: I think I preferred the second one, though the first one has more "basic" monsters. The second one is in color and is a real book though, instead of the stupid separate sheets thing.
Oh, and whatever you do, stay away from Black Flames (not to be confused with Black Spine). It's a bad, bad adventure that introduces a 22nd level dragon (defiler/psionicist) that apparently is "a minor player in the scheme of things".