There are bikes all over the city. Especially now that there are public rental bikes peppered all over the place, but they're already fairly common.

There are lime green bikes which you can rent for $1 using a cell phone. Rental unlocks a wheel lock on the bike and it remains unlocked until the renter re-locks it. So that's $1 for as long as you want the bike.

They've only been around for a few months, but they seem to be widely popular, given that I see people using them everyday.