Pay per click, or PPC, is an advertising technique used on websites, especially search engines. Pay per click advertisements are usually text ads placed near search results; when a site visitor clicks on the advertisement, the advertiser is charged a small amount. Variants include pay for placement and pay for ranking. Pay per click is also sometimes known as Cost Per Click or CPC. Pay per click advertising (PPC) is a search engine marketing technique that requires you to pay a fee every time someone clicks to your website from an ad you've placed in a search engine's results . The more you agree to pay per click (or bid) for a specific keyword and the more effective your ad, the higher your site will rank in the paid search results

Overture
http://www.overture.com/
Overture is the oldest major paid placement search engine. It distributes its listings to a wide-range of search engines, including that of its owner, Yahoo (see the Who Powers Whom? - Search Providers Chart for a rundown of major partnerships). Overture launched as GoTo in 1997 and incorporated the former University of Colorado-based World Wide Web Worm. In February 1998, it shifted to its pay-for-placement model. The company changed its name from GoTo to Overture in October 2001. It was purchased by Yahoo in October 2003.

Espotting.com
http://www.espotting.com/
Major paid placement service with wide distribution in the United Kingdom and Europe . It merged with FindWhat (below) in July 2004 and operates as a division of that company. FindWhat.com
http://www.findwhat.com/
Find What is an important paid placement service in particular because the company has distribution on many major meta search engines
LookSmart LookListings
http://listings.looksmart.com/
Provides both PPC-based paid inclusion listings and more traditional PPC-based paid placement listings that have wide distribution on second tier search engines and meta search engines