Recently these engines have become extremely popular. The most popular is Overture (formerly goto.com). With these, you don;t actually pay to submit. Instead, you create an account and bid against other webmasters for a position in their index on specific keywords. In other words, instead of getting a higher position by the relevancy of your web pages to a keyword search, you get placed higher as long as you make a higher bid on the keyword than someone else. To be number 1 on a keyword search, you only need to be the highest bidder. The second highest bidder gets number 2, and so on. Each time someone does a keyword search that you are bidding on, and clicks on your link, you get charged the amount of your bid.
Like the other search engines, some of the paid placement engines have
agreements with other engines. So, being number one on a paid placement engine
may make you number one on a completely unrelated engine as well. But remember,
the moment someone outbids you, your position falls.