Google provides a free keyword research tool in support of their Adwords pay-per-click program.
Not necessarily the most information-rich resource, but it's free
The Keyword Tool generates potential keywords for your ad campaign and reports their Google statistics, including search performance and seasonal trends.
The tool is actually intended to help you for yor Adwords campaign but it’s also very useful to discover keyword for your organic search engine optimization
Other resources for your keyword discovery analysis are
Please use our free Keyword difficulty analysis tool to measure the real difficulty for a keyword to perform on search engines.