For your site’s search marketing research, one of the best sources of information for keywords and phrases you’ll want to work is competitive intelligence—what keywords your online competition’s optimized for, the ones that trigger search engines into sending them traffic.
SpyFu is probably the best-known tool for that.
Most of its power isn’t free, but its basic free functionality can give you a useful quick snapshot of how your competitor’s site is performing with the search engines and what they may be up to SEO-wise. You don’t even have to register; just enter any URL, and SpyFu will basically tell you how much search engine traffic its getting and for what keywords (at least the top 5 or so, with a free query). It even suggests similar websites you may want to research as well.
Great stuff for when you’re starting out, and when your numbers are starting to get really serious, SpyFu’s paid options are frankly a bargain.