Advertisers who use Google's AdWords service to place advertisements on Google's search engine results pages (SERPs) have access to a great tool for doing keyword analysis. Luckily, the Google AdWords Keyword Tool is also available for free to the general public.

The AdWords Keyword Tool has two ways of generating keywords. The first is to have you seed the tool with one or more keywords (or keyphrases). After entering in your keywords, Google returns the keywords it thinks are the most relevant to your seed keywords. This includes so-called lateral matches -- keywords that are related by meaning but not necessarily by root words.

The second way to generate keywords is to give the tool the URL of an existing page. It analyzes the page and generates a list of relevant keywords based on what it thinks the page is about. (This is a great way to spot check what Google thinks your page is about!)

The AdWords Keyword Tool gives you more than just lists of keywords. It tells you how popular (on a simple scale -- no exact results) each keyword is in Google, the average cost per click for a keyword, and how much advertiser competition there is for the keyword. The lack of absolute numbers is frustrating, but the AdWords Keyword Tool is still an invaluable tool.

The Google Suggest Explorer is a free keyword tool written by myself for quickly finding long tail variations of root keywords using Google's own search query completion service.

The term long tail in this context refers to infrequently searched but very specific keywords. It's derived from the more general concept of The Long Tail, first popularized by author Chris Anderson. See his explanation of The Long Tail for more details.

The Google Suggest Explorer is very simple to use. Give it a keyword or even a partial keyword and it returns you a list of long tail suggestions that start with that keyword. For example, a search for mortgage yields long tail variations that include the following:

• mortgage amortization
• mortgage refinance
• mortgage payment calculator
• mortgage broker license
• mortgage amortization tables
• mortgage insurance

The tool also finds related variants like florida mortgage interest rates and california mortgage. You can also find information about the individual words in a phrase.

The Google Suggest Explorer makes it easy to "drill down" on keyword variations. It's also fascinating to see what others are searching for using either the recent queries or top queries pages. (Please note that these pages may contain adult-oriented queries on them.)

Although Google's keyword tool doesn't provide search volume data, the Digital Point Solutions Keyword Suggestion Tool helps fill in some of the gaps when you want to know exactly how many people are searching for a specific keyword.

The Digital Point Solutions Keyword Suggestion Tool is a "meta" keyword tool that combines keyword suggestions from Wordtracker (a commercial service) and from Overture (Yahoo's advertising program) into a single, convenient report. Using this tool you can get estimates on the number of times a day users are searching for a given keyword or phrase.

Note that the results shown by this tool are just estimates. For competitive search terms the estimates can be quite low because they only take into account Wordtracker's data (which is collected from various second-tier seach engines) and Yahoo's data. Given that Google's search volume is much higher than Yahoo's, you can expect the actual search numbers across all search engines to be significantly higher. Experts suggest a multiplication factor of anywhere from 4 to 10 times the Overture data, but there's no real consensus. Estimates for less-frequently searched terms aren't as dramatically different, however.

The list is far from complete... bookmark this lens and keep visiting as I add more free keyword tools!