The first step in registering a domain name is checking to see if the URL you want is available. These days, you have to get pretty creative with your naming, as one can tell by reading about all the oddly named Web 2.0 start-ups that are popping up.
Here's a utility that makes the lookup fast easy: Squurl. Just type in the name you're thinking about, and as you type, it returns "taken" or "available" for your dot-com, dot-net, and dot-biz domain. It also links you to popular registrars.
That's it. Nothing fancy. This will probably be a feature within the registrars themselves shortly. But for now, it's fun and useful. And with Squurl, it's easier to explore potential domain names you're thinking of than it is to do the old back-and-forth you have to with other lookup utilities.