Thanks for the plug, Enrious!
Keeping away from my own product, let me give my "sleepers:"
Beginner's Adventure:
The Wizard's Amulet - Don't hear too much about it any more, and yes it IS linear, but all of the sidebars and explanation were really great to people first picking up 3rd edition (which, two years ago, WAS all of us).
Electronic Adventure:
The Lost - If you can find it anymore (I have it on my computer). This was a beautiful free product that was put out in the early days of 3e and disappeared a couple of months later.

Does a great job with atmosphere and an even better job showing you how to make "mundane" magic items unique. The +1 undead-bane longsword named "the Blade of Adellas-Kai" was the favored weapon of my party's fighter for literally months and a dozen levels after this adventure because of its coolness value (not game mechanical value).
Print Adventure:
Nature's Fury - this one is a lot tougher, but I have to give the nod here to Nature's Fury. I liked it a lot.
Electronic Rules Supplement:
Races of Evernoor, Vol. I - While I appreciate Enrious' plug for the Enchiridion of Mystic Music, I said I wasn't going to pimp my own stuff (I guess I just did) but I'm going to go with the vastly underrated (IMO) Races of Evernoor. I liked Templates: Bodies and Souls, and I liked Monte's, Ambient's, and Natural 20 press stuff, but they've gotten too much pub to be considered "sleepers." Close runner up is the DM Graphic Cyclopedia - lots of good ideas, but poor to middling OGL compliance (at least the version I have).
Electronic Aid:
DM Dungeon Designer - A remarkable little gem. Simple, elegant, and completely under the radar. Campaign Suite, PCGen, Jamis Buck's stuff, E-Tools - all get too much pub for this one.
Print Rules Supplement:
Stronghold Builder's Guidebook - The overlooked Splatbook. Too bad, really... has good stuff.
More if/when I think about it.
--The Sigil