CycleTrader is the best website for used bikes,
They let you limit your search by year, price, and/or keyword.
What type of bike do you prefer?
- Cruiser style, (Classic, Harley style - Harley, Honda Shadow, V-star, ),
- sport (CBR, GSXR, YZR, Crotch Rocket, Racing),
- dual purpose ( On+Offroad, Enduro - Honda, bmw... and more)
- Sport Touring (Long Haul, luggage racks electric seets, windshield, radio - Honda Goldwings, BMW's, Harley Road King,)
If you want a fast look at a bunch of bikes,
Cyclebytel, or
Cyclebytel Classifieds used to have a good interface to look at the new model years, but they are in the middle of changing it.
Also for Bay area peoples
Craig's List is a pretty good free listing site, never looked there for bikes though.