Keyword Tools

One of the first things every web site owner should do is research keywords using tools provided by the major search engines. These tools can tell you the popularity of keywords and help suggest additional keywords that pertain to your site.

Here is a list of some online keyword tools:

• http://tools.seobook.com/general/keyword/
  Billing itself as the "Coolest Keyword Research Tool Today", this site does a great job of aggregating information from most of the sites below on a single page.
  

Google Tools

• https://adwords.google.com
• http://www.gorank.com/seotools/ontology/index.php
• https://adwords.google.com/select/TrafficEstimatorSandbox
• http://www.google.com/trends

Yahoo / Overture

• http://inventory.overture.com

Others

• http://www.quintura.com/
• http://freekeywords.wordtracker.com/
• http://www.keyworddiscovery.com

Know of any other great tools out there?