Yahoo is the second biggest search engine and directory around the world! Yahoo also supplies results to Altavista, Inktomi, and AllTheWeb search engines. Having in mind all that, you shouldn't miss the opportunity to submit your site to Yahoo search engine.
Note: Sometimes submitting your website to Yahoo is not enough to have it indexed. The best way to be sure that your website will be indexed is if it is linked by another website that has already been found by Yahoo. Thus, when Yahoo robot crawls the other website, it will follow the outgoing link to your site as well and index you site.