Google AdWords
https://adwords.google.com

The Google AdWords program places paid listings within Google's search results, as well as on some other sites that carry its listings.Winner of best paid placement service in the 4th Annual Search Engine Watch Awards.

Search Engine Watch's Submitting Via Paid Listings: Overture & Google AdWords page provides basic tips on getting starting with Google AdWords.

Search Engine Watch members have access to the How Google Works area, which provides in-depth coverage of Google's paid listings program. Learn more about becoming a member on the membership information page.

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. Overture was second place for best paid placement service in the 4th Annual Search Engine Watch Awards.

Espotting.com
http://www.espotting.com/

Major paid placement service with wide distribution in the United Kingdom and Europe. It merged with FindWhat in July 2004 and operates as a division of that company.

FindWhat.com
http://www.findwhat.com/

FindWhat 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.

Kanoodle
http://www.kanoodle.com

The Leader in Content-Targeted Sponsored Links.

Kanoodle provides advertisers an unparalleled opportunity to leverage text-based, cost-per-click sponsored links to reach their customers on the Web's search engines and content destinations. Kanoodle's products -- ContextTarget™ for content-targeted sponsored links, LocalTarget™ for locally-targeted sponsored links, BehaviorTarget™ for behavioral-targeted sponsored links, and KeywordTarget™ for search-targeted sponsored links -- are distributed on networks of high quality search and content providers, including MarketWatch.com, MSNBC.com, USAToday.com, CNET's Search.com, the InfoSpace search properties and on Kanoodle.com. Kanoodle was founded in 1999 and has offices in Amherst, NY and New York City.