Nightmare Keep
ANY Ravenloft Module after they did the Grand Conjuction (And the Grand Conjunction modules sucked too)
Ravenloft MC2
The Ravenloft Black Book
Ravenloft 3.0
The Epic Level Handbook
Players Options (ANY OF THOSE 2.5E DOGS)
Most the the FR# books.
The old 1E XXXXXX Geomorphs
Synnabar.
And I'll get a lot of flack for this...
THREE POINT FIVE!!!! Next time, can I get a reach-around on the $300 in books I bought?
Book of Vile Darkness (Should ahve been entitled: WotC tries to scare a nun)
Most Dragon Magazines for the last few years
The Complete Racial books...
The old Forgotten Realms hardcover book (I gave that thing away to someone on mIRC)
Most of the B series modules after 5
The X series modules after 1
The Lost Caverns Of Tsucksomuch
Maztica
The Horde
Most of the Oriental Adventures Modules
A few of the Ravenloft Isles in the Mist were useful, but the rest were all crap.
The Villians Lorebook
Of course, everyone here is bashing that splatbook Hero Builder's Guidebook, but a lot of you are forgetting something... Some people come to D&D with NO IDEA of how to play. Sure, teaching them works, but many people got some worth out of it. Many of my new players think it's good, and I don't make fun of them for liking it, or for using it. Jeez, get off your high horse and remember, some whole groups are made of newbies. My daughter and her friends used it to great effect.
Oh, and worst 3.5 products?
Don't know, quit buying stuff unless it was 3rd party d20 stuff.