Overture Keyword Selector Tool:
This is a good first step to generate a basic keyword list of phrases containing the actual term you entered. Yahoo! has indicated that the search numbers are old and inaccurate so don’t count on exact numbers. However, it’s a useful tool to determine if a keyword is being searched for at all.
Good Keywords: (Free Download)
This free download for Windows users is similar to the Overture Keyword Selector Tool. It also has a feature that identifies common misspellings of words.
Google AdWords Keyword Tool:
Enter a search term or terms into the ‘Keyword Variations’ section, and you'll be shown other keywords that contain that term or are related to that term (synonyms). You can then sort keywords in terms of search volume popularity on Google. Google does not provide actual number of searches.
You can also use the ‘Site-Related Keywords’ section of the tool to identify the keywords of an existing web page by entering the page URL.
Wordtracker Free Keyword Suggestion Tool:
Wordtracker has a robust fee-based keyword tool that’s popular among the SEO crowd. In addition, this free tool is useful for building a list of synonyms. Enter a search term and you'll be shown 100 potential keywords related to (and including it), ranked by daily popularity. You can also sign up for the free trial of the paid version (recommended) to dig deeper into related keyword terms. This is a great way to identify secondary keyword phrases.
Microsoft AdCenter Labs Keyword Group Detection Tool:
This tool will also build a list of synonyms.
Trellian Free Search Term Suggestion Tool:
This tool provides synonyms as well and is very similar to Wordtracker’s tool above except that it ranks the keywords by annual searches as opposed to daily searches.
Bruce Clay’s SEO Tool Set Traffic Check:
Use this tool to produce an estimate of daily search activity for a specific keyword phrase across the major search engines broken down by Google, Yahoo and MSN. This one will provide actual numbers (but they are still estimates).
SEO Book Keyword Suggestion Tool:
Provides aggregated data from the free Overture, Google and Wordtracker tools listed above.