There are hundreds or even thousands of search engines exist today, and many search engine optimization companies promise to list your website to most of those search engines if not all. Even though there are thousands of search engines in existance, only a few are worth considered. Applying Pareto's Principle 80-20 Rule, the final four below will cover well over 80% of all search traffic. The pie chart shown below depicts current market share of each search engines listed as of July 2005. Considering the fact that other search engines such as AOL uses Google for their results, the incumbent Google has more than 50% of all search engine market share.

Source: comScore MediaMetrix (7/2005)

Google

Yahoo

Dmoz

MSN

All the rest are driven from the results provided by the final four search engines.