Decipher's Star Trek RPG and Romulan setting. Seemed like a good idea at the time, but we quickly realised that Star Trek doesn't really work as an RPG. Makes a good setting for a collective writing group, but not a tabletop game.
d20 Oriental Adventures & Rokugan setting. Bought them when i thought a guy in our gaming group was going to run the setting, but he never did. No use for them whatsoever. Don't think I've cracked the books since my first readthrough.
Most of my FR collection (FRCS, Magic of Faerun, Races of Faerun, Lords of Darkness, Silver Marches, Unnaproachable East, Faiths & Pantheons, City of the Spider Queen). Bought back when I was running D&D. I don't completely regret them, but the books are mostly useless to me now and don't even offer the enjoyment of reading value.
Savage Species. Not a bad book, but really not a necessary book and could have used the money elsewhere.
Draconomicon. The last D&D book I purchased. I commonly refer to the DM's chapter as '100 ways to TPK a party.'
Now, there are books I get no use out of, but love them for their reading value. I'll never use my Farscape RPG or my Mummy: The Ressurection, but they are fun reads.