Instant Domain Name Search uses Ajax to instantly find out whether a domain name you enter is available. It only searches for .com and .net domains, and an available domain gives links to buy it at either Yahoo or Register.com. A better feature would be to allow you to choose which domain registrar to link to when a domain is available. Still, it’s a useful site.

And I would link to the place I found this if I could remember. Searched all over and can’t for the life of me find out who blogged about it besides Steve Rubel.