Oodle.com is a federated search engine for classified ads – it does not (at least as far as I know) have its own ads, but act as a portal to other ad sites, presumably in return for a share of profits.
The value created is partly from the interface technology (enter "Mercedes 450 SEL 6.9" and it knows you are looking for a car and format the page so that you can drill down on models and years) and partly in that it accesses all kinds of local and community-based listings.
When I was looking for my used Mercedes I searched large advertisers such as cars.com and autotrader.com – but they only show their own ads. Oodle.com would have found me more cars (though not any I would have bought rather than the car I did get.) Useful because many markets are local and therefore hidden if you come from outside.