Major Pay Per Click Engines
There are only two "tier one" PPC engines: Google AdWords and Overture. As of this writing, Google AdWords' most significant partner is AOL. On the other hand, Overture can get you traffic from Yahoo! and MSN. Some people speculate that Google's traffic is more information driven while Overture has a greater consumer-oriented traffic, but we will leave that to your tests to decide.
Besides the Big Two in Tier One, there is a Tier Two. Here you will find:
For reviews of these Tier Two PPC's, visit PayPerClickSearchEngines.com.