Pay Per Click Search Engines

Enhance.com (ah-ha.com)
Based on quality traffic and a very friendly and helpful Customer Service Department
ePilot.com
Based on a good amount of traffic (over 200 million monthly searches) and one of the largest distribution networks.
Espotting.com
European PPC Engine. Espotting is a progressive PPC Search engine that has developed some key partnerships in the UK. They are actively working on developing partnerships in Europe to increase their coverage.
Find What
PPC Engine. One of the "Top" pay per click solutions based on value per click, ease of advertiser usability and most importantly targeted traffic. Serving over 22 million searches a day through such affiliates as CNET's Search.com, Excite.com, Webcrawler, NBCi, MetaCrawler, DogPile.com, Go2Net and Microsoft Internet Explorer Autosearch.
Google AdWords
Googles own PPC program. One of the "Top" pay per click solutions based on a strong brand and one of the top destination site that people know/trust. Plus a substantial amount of traffic - not only from their own sites but also through affiliates like America Online, Ask Jeeves, Earthlink, Lycos Canada, Netscape, Compuserve, AT&T Worldnet and NYTimes.com. Most recently added distribution through the Walt Disney Internet Group Web Properties - Disney.com, FamilyFun.com, Go.com and Movies.com.
Kanoodle
PPC Engine. Based on a fair amount of quality traffic to top-tier (highly searched) keywords- a good ROI and a friendly, helpful and readily available customer service department.
LookSmart
PPC Engine - Based on a big distribution network. Uses distribution partners like MSN, Alta Vista, About.com, Netscape, CNN.com, InfoSpace Network (Dogpile.com, Excite.com, MetaCrawler.com and Webcrawler.)
Overture
Yahoo's PPC Engine. One of the "Top" pay per click solutions based on amount of traffic through affiliates like Yahoo.com, MSN, Excite.com, Altavista.com, MetaCrawler.com and InfoSpace Network (Dogpile.com, Excite.com...etc.). They have a great reputation with a friendly advertiser system that now is offering "Auto Bidding" which allows you to cap bids on each term individually.
Sprinks.com
Based on a good amount of qualified traffic through About.com's GuideSites, plus affiliate distribution partners like CNET.com, Metacrawler.com, Mamma.com, Forbes.com, Excite.com and DogPile.com ...and still, fairly low bids.
7Search.com
Find what you are looking for with this bid for placement search engine.