I nearly flipped my lid when I first used this plugin for Firefox. It's a sidebar that ? at the push of a button ? scans whatever page you're on and gives you a read on its keyword volumes and keyword density. It also provides this information as a natty, visual tag cloud for those with autistic tendencies like myself. You should use it for snooping on competitors. If they're good at their game, you'll learn heaps about why. I promise it'll blow you away.
Google Adwords Suggest

Just type in whatever keywords and/or phrases you're investigating, hit a button and this tool will tell you how many people have used the same verbage to search Google in an average month, and also how many competitors are out there bidding on the same terms as part of their PPC ad campaigns. When it comes to content planning, I'd be lost without this one. It tells me whether or not I have an audience for my keywords, and whether there's any competition out there. As such, it helps me know what content to write, how to do it and how to promote it.
Wordtracker

Like Google Suggest, but provides (independent) data on keywords from a wider variety of search engines. Basically, it keeps a three month store of activity on Google, MSN and Yahoo, and tells you how many times a day your keywords have been used by the world at large. In addition, it gives you a superb competitive index that tells you how many other interweb pages are optimised for your terms. This is gold dust as it enables you to get a true measure of whether a Content Optimisation strategy is worth pursuing or not. (What looks interesting from a search volume perspective may often turn out to be a monster when it comes to competition!) Costs a couple of hundred bucks a year.
Competitive Online PR