Overture and Google adwords are the 2 big PPC.
Some others are Kanoodle, 7 search and findwhat. They vary quite a lot on traffic quality and quantity and also depending on what you are promoting.
Have a look here for some good discussions about the various PPC:
http://searchengineforums.com/bin/Ultimate.cgi
http://www.webmasterworld.com/forum33/