CarFax

Word of warning - not everything shows up on a CarFax. I had over $6K of damage done to one of my cars, fixed at dealers, filed with insurance, the whole shebang. When I looked at the CarFax 2 years later, it was clean.

Ditto with 4 cars I was looking to buy used back in 2002. I paid $90 for an independent mechanic to look at them and all but one had been in frame damaging accidents. CarFax clean on all of them (4th one was from the NE and had rust).

CarFax will show if there's a salvage title and sometimes you can read between the lines (fast sale after purchase) that it was bought back under lemon laws, but be sure to have whatever your buying (or even selling - Carmax saw my $6K damage immediately) checked out by someone who's not involved in the transaction.