Google AdWords started it all.
In my book, it’s still the king. If you’re not including AdWords in you monetizing scheme you missing the boat. It changes all the time. Go right to the horses mouth to learn more about AdWords

Keyword and inclusion targeting Minimum keyword is 10c

Refine you campaigns on Google and then try them here

Fastest growing and cheapest

Serves over 25,000,000 search a day.(25 Million)

Distribution:  Search.com Dogpile.com webcrawler.com excite.com  

Overture is the second biggest PPC engine. It’s you need to learn how to use it’s “KeyWords Analyzer”  
They move it around so you may have to search for it’s new location. Try here first Overture KeyWords Analyzer