Which Search Engines Are Important? Also see How do I increase my search engine rankings?

Not all search engines are equal
Don't believe sites that insist you should "Get registered with hundreds of search engines!" You don't need to waste your money or time. Instead, get listed with the top few and the rest, if they're good, will find you. If they're not, they'll go out of business so it wouldn't have mattered anyway.

The top search engines and directories you should register your site with are:

Google (includes AOL, Hotbot, Netscape, Excite)
Yahoo (includes AllTheWeb, Altavista, Inktomi, Lycos, Hotbot)
MSN

Directories (get listed here and it will count towards your incoming links that Google will use to rank your site):

Open Directory Project
www.joeant.com
www.skaffe.com
www.bluefind.com
www.zeal.com
www.goguides.org
www.gimpsy.com
www.wowdirectory.com
www.sevenseek.com
wikipedia

Google Local
Yahoo Local
MSN Local
Ask Jeeves Local
AOL Local
Amazon's A9

Internet Yellow pages - also get listed here for local searches:

Superpages.com
Yahoo Yellow Pages
AOL Yellow Pages
MSN Yellow Pages
YellowPages.com
Switchboard.com
Qwest Yellow Pages
Bellsouth Yellow Pages
Smartpages.com
Yellowbook.com
Citysearch.com