Excellent domain search tool, ajaxified, and supports multiple (even configurable) extensions, probably not 100% reliable though if it relies on whois.
Search across all the funky domains, short domains, etc. Awesome, instant, domain search. :-)
Wild West Domains is the reseller arm of GoDaddy. Same company (I think) but WWD allows you to setup a whitelabel domain reseller system.
The dot-o-mator allows you to punch in a bunch of keywords (or it has predefined lists) and combine them to make domain names. Then with one click you can check the availability of all those domains. Very useful.
Dotster offer white label domain name registration and hosting services. The interface is fairly slick. Could be useful.
A great tool to search domain names. Type in as many words as you like and the results appear below almost instantly via sweet ajax interface. Very nice.