Probably the most famous, and most used, keyword research tool is Overture's Keyword Selector Tool. (Which, despite recent reports and confusion about it's demise, is going to stay online.) I've heard some people say it's great, and some people say it's garbage. But however much you value the data, you have to agree that it isn't perfect. So here are 5 alternative keyword databases for you to use for overture's supplementation or replacement. (And as a bonus, they're all free):

Keyword Discovery -- Recently released a free tool that outputs the top 100 terms. More keywords are available for a price. Search counts seem to be about 5 to 20 times what overture reports.

Word Tracker -- Also released a free version a week or two ago that generates the top 100 terms. Somebody set up a site that gets the information and lets you export it to a csv. (But it's not done yet.)

Google Trends -- Doesn't include search counts, but you can compare it against other stable keywords to get an estimate. Could have the largest database, but they probably don't use all of it.

7Search -- Actually returns several times the number of keywords that Word Tracker does. 100 terms per query.

Gigablast (example search for seomoz) -- Reports search counts as percentages. I'm not sure exactly what the percentage is of, but the tool could be useful for finding related keywords.

Any other good ones I'm missing?