If it's just old stuff that I'll never use -- 26mm road bars, 25.4mm mtb bars, short seatposts, OEM saddles, Look pedals, and the like -- it goes to the bike co-op. Anything that's current but unwanted (alt bars, cartridge bottom brackets, wheels, tires, road brakes, 9-speed cassettes) goes on Craigslist. Anything current and useful, I keep until it falls into one of the other categories. 90% of the stuff I think I'll use one day eventually finds a new home.