Some setting love:
Planescape 3.5: As mentioned above, setting information, planar feats, new prestige classes, and post-Faction War Sigil. Include important NPCs, plot hooks, and such.
Star*Drive for d20 Modern: Give us the d20 stats for those Alternity aliens. Give us cybertech rules. Maybe update the timeline... Such a great setting. It's too bad it got dropped with Alternity.
Of course, with these two, I could finally run that epic Planescape/Star*Drive crossover campaign I've been puttering with for years.
Dark Matter d20: Same thing. This setting rocked far too hard to be abandoned.
A free PDF from Fantasy Flight that updates their Dragonstar setting-specific rules for D&D 3.5-- things like the Ranger's combat styles-- and includes optional rules for playing the Dragonstar setting using d20 Modern rules. Maybe even some hints as to the Dragon Empire's connections to other planes, and the cosmology behind their version of the Prime...
A Wild West supplement for d20 Modern that includes era-specific costs for items, a tech timeline, and optional rules for Western variants like Western/Steampunk and Weird West. (Suggestions of Deadlands d20 will be met with horrible, screaming doom.)
Shadowrun d20, keeping their magic/fatigue system (and other setting-specific rules) but using d20 mechanics. I love the setting, but the system makes my soul bleed.
A complete campaign setting for d20 Modern/Four Color to Fantasy, complete with d20-specific elements (mind flayers and fraal as aliens) and an energetic everything-but-the-kitchen sink universe that incorporates magic, psionics, mutation, and the amazing powers of gamma radiation. (I am doing this, but do not fancy myself a publisher and could never get the appropriate permissions)
A primitive/Stone Age D&D campaign setting, complete with dinosaurs and a mammal/reptile rivalry. Primitive humans and halflings fight for survival against rampaging dinosaurs, brutal nature, and the fierce competition presented by orcs and goblinoids. Lizardfolk rule the swamps and Thri-Kreen the deserts, but survival creates the strangest allies...