I'm assuming you mean what PPC advertisers do affiliate marketers utilize??? The only two I primarily use are
Yahoo Search Marketing (formerly Overture) and
Google Adwords. There are a slew of other smaller PPC engines that should not be overlooked though. Here are some of the leaders:
AzoogleAds
MIVA
Go Click
Enhance Interactive
7 Search
Kanoodle
ePilot
Search 123
SearchFeed
I have not used these additional PPC engines but they've been around for quite some time. I have used Searchfeed Publisher with moderate results though. Hope that helps.