(Updated March 2013)
Good Keywords KeywordPad – KeywordPad is a Windows software for managing large keyword lists used in your Internet marketing campaigns (organic search engine optimization, PPC advertisement campaigns, etc). You can also use it to generate long-tail keywords. Free download.
Google AdWords Keyword Tool – Use the Keyword Tool to find keyword ideas, estimate traffic for existing and new keywords, and identify irrelevant terms to use as negative keywords. Find keyword ideas based on a specific keyword, a web address, or a keyword category.
SEOBook Keyword Tool – Great for cross-checking, provides side-by-side results for Google, Wordtracker, Bing, plus links out to additional analysis tools.
SpyFu – Keyword Tools for SEO PPC.
WordPot – Free web based Keyword Generator for finding niche keywords.
WordTracker Keyword Tool – Free keyword suggestion tool for SEO.