Major Search Engines: According to the Comscore Media Matrix, in July 2006, five search engine networks drove 96.6% of the searches in the United States. Google's network alone is responsible for more than 43% and Yahoo's network is repsonsible for an additional 28.8%. The full results are listed below:

Google - 43.7%
Yahoo - 28.8%
MSN - 12.8%
AOL - 5.9%
Ask - 5.4%
Other - 3.4%

Google: Shows searches at any Google-owned web site, excluding AOL and timewarner properties.
Yahoo: Shows searches at any Yahoo-owned web site including those of Alta Vista, AllTheWeb and Overture.
MSN: Shows searches at any MSN owned site such as MSN Search.
AOL: Shows searches at all AOL or Time Warner-owned web sites, including AOL Search and Netscape Search.
Ask: Shows searches at Ask.com and any site within the Ask/Excite Network, including Excite, iWon, MyWay.com and My Web Search.
Other: Shows searches that occur at other search sites.

Other Resources:

I help you Forums - White hat SEO forum.

Searchengineforums.com - More forums with a white hat lean.

Keyword Density Analyzer: This is a great tool that can help you determine the keyword density of your site versus your competitors.

Overture Keyword Suggestion Tool - Helpful tool for determining your target keywords and key phrases.