I knew from the moment I saw Carsabi that it was going to be big. Buying (and selling) used cars is such a pain in the ass that it really surprises me nobody has done something as simple and clean as them.

I also like how the initial listings are populated by Craigslist, but on each panel is laid out cleverly to display the "source: craigslist" as a changeable element. This allows them to piggyback on the volume of listings on craigslist, while subtly suggesting that future listings may come from any source (perhaps maybe an in-house ecommerce system?)