Focus on the Big Three Search Engines, Google, Yahoo and MSN (live.com) as they comprise roughly 90% of all searches in the U.S.
Don't be fooled by offers to "submit your website to the 1000 top search engines" as there are not 1000 top search engines. Many of the smaller search engines and directories actually draw their search results from the Big 3.
*Source: Nielsen Online reports December 2007 data for the Top U.S. Search Providers. Top search providers, ranked by total searches. Searches represent the total number of queries conducted at the provider.