http://nsaltsman.com/recommends/ppcwebspy
http://nsaltsman.com/2009/02/ppc-web-spy-review/

Pros:
* Steal adwords keywords from anyone
* See the ad position, avg CPC, max CPC, clicks/day and price/day for each keyword
* See the destination url of each ad
* Easy to use

Cons:
* Only works in Firefox
* Doesn't work for all ads (also a good thing, see video)
* Free version is limited to only 10 keywords per ad (less info)