SpyFu

SpyFu focuses on competitor research for search marketing. Enter a domain and it lists the keywords that site has bought ads on, the ads it has run over time and an estimate of its monthly budget.

The ad history is the part PPC managers use most: you can see how a competitor's headlines and offers changed over the years, which keywords they kept and which they dropped. Keywords a competitor has paid for over a long period are a hint that those terms produce customers for them.

All numbers are estimates from SpyFu's own data, so use them to compare and plan, not as facts about another company's spend. SpyFu also includes keyword research and a negative keyword suggestion tool, and it covers organic search data for the same domains.