GoTo (http://www.GoTo.com) is a search engine that allows advertisers to "bid" on search results. Each Web site that wants to be displayed higher in the search results for a particular keyword bids a certain amount of money per clickthrough. The highest bids are displayed first on the page, followed by lower bids, followed by non-bidding matches. Every time a user clicks on a link, GoTo charges the advertiser the amount of their bid.
The end result is a search engine that works more like the Yellow Pages, displaying matches to high-bidding commercial sites before any matches by content. A sample search for "linux" turned up hits for Linux servers, Linux training, Linux toolkits... and finally, on the second page starting with match #72, links to Linux news, documentation, and distributions. In contrast, Google counts the number of links to a Web page to determine how high to display a match, resulting in a list sorted by how many other Web users consider a site useful. They're not at all sneaky about this model; in fact, they promote it right on their front page and list the amount each advertiser has bid right on the search results page.
Despite their cynical techniques, GoTo is a success in the business sense. Google is the number one search engine because it's so good (and is fully integrated with Yahoo!), but GoTo is number two because they've partnered so widely, have a paid affiliate program, and are actually profitable.