Sometimes it’s quite useful to have an idea of what your competitors are doing on Pay Per Click, what keywords they’re bidding on, what they’re spending, stuff like that.

Disclaimer: Neither of these tools provides a completely accurate picture but they’re both very useful:

http://spyfoo.com/ – Gives you a list of major keywords your competition is bidding on, an estimate of their daily budget, data about their organic listings, average bid position, and how many competitors they have. Rather insightful.

http://www.googspy.com/ – A related technology, but presented in a different way. The keyword list it generates is incomplete but quite helpful.

http://www.lexfn.com/ – Type in a word and it gives you related terms and concepts.

Thanks to Perry Marshall for this info