Mouser Electronics
http://www.mouser.com
Various electronics
W.M. Berg
http://www.wmberg.com
Various gears, sprockets, bearings, etc.
GlobalSpec (Registration Required)
http://www.globalspec.com
Not a supplier, but a huge database of suppliers. Register and you can enter specs for a part and see who has something that fits. Warning: If you click on links to parts you've found, you'll be getting catalogs in the mail.