SEO-Book.com’s Rank Checker – A popular free tool that will analyze your rankings for keywords you input and track them if you wish.
SEO Books Keyword Suggestion Tool – The reason I like this tool so much is that it gives you the spread, so to speak. As anyone who has done any amount of keyword research knows, the source of your information greatly influences the numbers you get back. Well this tool shows you numbers from Google’s Adwords Tool to Wordtracker suggestions, Bing, Yahoo and a number of others to give you a breadth of coverage and can compare relative volumes based on numerous data sources.
Bruce Clay SEO Toolset (Free Tools) – Very scientific, data driven, heavy emphasis on keyword density in titles, page elements based on data from the top results for a given keyword.
SEOmoz Free Toolbox – A collection of handy SEO Tools that rival anything else out there. Check it out, plenty of gems in there.