For many people Google AdSense is the best.
Here are five others. Most people don't have results with these that are as good as AdSense results, but there can be exceptions:
Yahoo Publisher Network
Yahoo! Publisher Network
BidVertiser
BidVertiser - Pay Per Click Advertising On Sites Of Your Choice.
Chitika
Chitika, Inc. » A Blog Advertising Company » Home Works best on product sites
Kanoodle
Kanoodle - Providing Search-Targeted Sponsored Links Since 1999
Kontera