You can enter detailed search criteria (including manufacturer, model, kilometres on the clock location), or use the Quick Grab buttons to get all the vehicles in a given category. The site doesn't have a direct list of which other sites it indexes, but Drive, Carbuddy, Countrycars and Trading Post all appear to feature. I didn't spot any links to Carsguide or CarPoint, which are two of the largest sites, but there's still a pretty broad selection.
Running an aggregation site isn't always easy, as the sites whose results get collected aren't always pleased to see that happen. Presuming it can dodge those issues, CarGrabber is a useful resource for prospective vehicle buyers. It's a free site, requires registration if you want to save search results.