Keyword Research is one of the most important phases in any Search Engine Optimization campaign. (tip ->>) Keyword research is the process of choosing the most most advantageous keyword phrases that can help users find the websites they are after.
You can consume unbelievable amounts of time optimizing your pages for better ranking in the big players (Google, Yahoo, MSN etc.) SERP's yet it is a fruitless effort if the right keyword phrases are not targeted. High search engine rankings due to good keyword selection may still not bring you significant traffic. So, identifying your target audience and research what keywords they might be typing in the search engines to discover your site.
SEO Fever has compiled a list of Keyword Research Tools (free and third-party vendor) that we find very useful. There are quite a few so let us know how the tools rank with you.
Wordtracker
Wordtracker provides a wide variety of ways for you to seek
out top-performing keywords.
Webmaster
Toolkit 
Simply enter the sort of word of phrase you wish to be found
under, and the tool will suggest some additional words and
phrases you can think about using.
Kwmap.com
Keyword Map for the whole Internet
NicheBOT
(Wordtracker keywords) 
Discover untapped keyword phrases in your niche market.
Overture
Keyword Selection Tool
Not sure what search terms to bid on? Enter a term related
to your site and we will show you.
Google
AdWords Keyword Tool
Get ideas for new keywords that can help you improve your
ad relevance. If you're running your ads on broad-matched
keywords, you may also find additional terms that may trigger
your ads.