Craigslist
SUCKS due to spam, 1996-era search, misposts, an overlimited posting format, a lack of keyword filtering (I could go on for days on why craigslist sucks. So could you, even if Craig gives us happy feelies) but they're really the only game in the bay area. Rent.com and Apartments.com have been a joke since their inception. Padmapper is alright, but definitely hampered by trying to sharecrop Craigslist without an api.
This market seems ripe for innovation and disruption. Before I jump into the game, I figure somebody else, or many other people are trying to build a better apartment search site. So, what's out there?