So, Twitter will be dropping "inactive" username soon and you are going to need a place to quick search to find available ones. So, I've created grabTwt.
The site is still in development, but you can see the primary search on the homepage and give it a whirl. It searches as you type and lists the available usernames.
My plan is to make it so you can save your list of usernames (login with OAuth works, but I haven't added the save functionality) and receive e-mail updates when a username you want is available.
Take a look and let me know what you think. If you like the project please follow it on Twitter @grabTwt.