Can we start adding Pathfinder stuff to the list yet?
Pathfinder:
Pathfinder Core - because sorcerers are cool now, m'kay?
Advanced Player's Guide - archetypes are a handy way of doing alternate classes and races.
Kingmaker - I am digging the exploration and nation building rules of Kingmaker, and the expansion thereon done in Book of the River Nations by Jon Brazer Enterprises . Nice, simple, and in a wilderness area the nation building can begin at fairly low level, 'bout the time they get their first fireball.
D&D 3.X:
Skull & Bones is excellent when crossed with Call of Cthulhu.
Corsair (soon being reissued for Pathfinder, I think.)
Advanced Bestiary and
Deluxe Book of Templates - yeah, I like templates, and both have some really good ones.
Heroes of Battle - yeah, it's WotC, but it changed the way I handle mass battles that are not player driven.
Classic Play Book of the Sea/ Seas of Blood - two books, with the former being an expansion upon the latter. I almost always have naval combat in my games, and these fill the role nicely. Open Naval Combat Systems and trade rules.
D20 Modern, with a Caveat - I use the stuff with Spycraft 2.0
Elements of Magic: Mythic Earth. Yum! The magic that I use for Steampunk.
Thrilling Tales! - ain't used it, but wanna.
Stand Alone:
Spycraft 2.0 (I have a dire feeling in regards to the upcoming 3.0... mostly because 2.0 hit my sweet spot, rather than thinking 3.0 will be bad - 2.0 is just soooo gooood.
)
True 20 - a good, solid, generic system building off of the D20 rules.
Fantasy Craft - there are parts of this that I really like, and one chapter that I really don't like. Try it out, lots of folks really like the part that I don't.
Chopped Up Into Little Pieces, Then Cannibalized:
Quintessential Fighter - the Open Mass Combat System
Quintessential Wizard - rules for Libraries
Call of Cthulhu - when I mix a little Cthulhu into other D20 games.
Classic Play Book of Strongholds & Dynasties - the Open Mass Combat System II
The Auld Grump