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The new Tweexchange lets you quickly find available Twitter usernames and web domain names (through GoDaddy) at the same time. The results appear fairly quickly, as you type the name in the search box, which gives you a nice side-by-side comparison of available names.
If you like, Tweexchange can notify you when a domain that is pending removal will be removed. You can also back order taken web domain names, and bid for domains that are for sale directly from the service. In a world where a Twitter username is almost equally as important as a web domain name (or a Facebook name, for that matter), Tweexchange makes it really simple and fast to find a name that's available on both fronts.