Some other options are Google’s Adwords keyword tool,(it’s free to use), located at https://adwords.google.com/select/KeywordToolExternal?defaultView=2

or Submit Express’s keyword suggestion tool and traffic estimator at
http://keywords.submitexpress.com/

and Spacky.com monthly keyword search volume tool.

You can find some really good info on keyword research athttp://www.wordtracker.com/academy/ , this is Wordtracker.com’s Keyword Research Academy with articles by the top people in Internet.

Checking to see if anyone is using key phrases to look for information on your topic will give you added traffic if you can work some of those phrases into your content, meta tags and link descriptions.
Make a list of questions about your topic and terms or phrases which you would use to search and also list the same using synonyms for the words in your original list, because others might use different words than the ones you are using. Run these through the online tools mentioned and make a list of any additional phrases which show up.
Do a google search on each of the terms in your final list and see if you get anything with fifty or less results. These will be your most profitable phrases or terms to use.
Anything with less than 20 results may be worth investing in an Adwordsaccount, if you can get listings really cheap and can make sales from these listings.
Finally, make sure that you have your site listed using Google’s Sitemap tool, it’s free to use and tells the googlebot where all of the pages that you want indexed are at. Find info on it at http://www.google.com/support/webmasters/bin/answer.py?answer=34575&ctx=sibling ,
you will have to sign up for a free Google Account at
http://www.google.com/accounts/ManageAccount first if you don’t already have one, but it is free! You can access the sitemap tool from inside your account.

I mentioned Yahoo having a keyword research tool previously, but in order to get access to it you have to have a Yahoo Search Engine Marketing account, which requires a $30 dollar deposit for pay-per-click which you can budget however you want, but still you have to put up 30 bucks.
You can check it out here and sign up here. They also require a minimum bid per click of $0.10.

Saving the best for last, right-click and download this informative PDF ebook, “The Keyword Jackpot“, it’s free.