Basically, you install the software and when you perform a Google search on any keyword, it places a "view keywords" button directly below any Adwords advertiser's listing.
If you click the "view keywords" button, you can see the keywords ANY Adwords advertiser is bidding on! Plus, a bunch of other very cool stuff.
I've never seen anything anywhere near this powerful, especially being free.
You can download it here.
Make sure you go there right away, because I don't know how long this application will be free.
P.S. Here's the link where you can get the free Adwords keyword tool again.