Keyword tools

There is only one good source of info on keywords. It's google. You can use the:

1. The Adwords keyword tool which gives you gives you average searches for a keyword (I think over the last year), last month’s searches, and the keyword competition.
2. Google trends which gives volumes of searches for a word or keyphrase for the last five years.
3. Look at the bottom of a regular search on google where they recommend related searches. For instance, if you search for the word math, at the bottom of the page, Google will suggest: math games - multiplication match - math facts - math test - math formulas - math fractions - pre algebra
4. The adwords keyword tool also can give you suggested of related terms.
   

Other tools that I use:

1. http://seodigger.com/ - you can find out for which keywords your site (and your competitors) rank high enough to be in Google Top 20.you can find out for which keywords your site ranks high enough to be in Google Top 20.
2. WordPot - It's new to me. It ranks related keywords like the old overture tool. It claims to compare different sources and produce a more refined set of data by filtering out bots and refining exact/near matches. To quote: We collect our data from a combination of the real time searches done on popular meta search engines (like Metacrawler and Dogpile) and results published by Google (zeitgeist), Yahoo and Msn. We are doing this 24/7 and have been collecting data and building our database for a number of years. We then aggregate the results together to come up with the numbers that you see on the site.