Domain research engine Domain Pigeon now enables users to search and browse unclaimed Twitter usernames in addition to domain names.
Much the same way domain name research works on Domain Pigeon, users can filter results to display recently added Twitter names, view them alphabetically, gauge popularity or see only names with three or four letters (there are over 17,000 remaining) although you must be a paid, registered user of the site (a one time payment of $39.95) for that priviledge.
Membership might provide you with valuable access to a lot of interesting information, but even the free service offers a lot of valuable data and insights and is worth checking out. Hundreds of Twitter usernames are added each day and there are quite a few that are available right now that might make for a good Twitter brand. One of the interesting features of the service is that you can see how many other people are also interested.
Here's a view into several available Twitter usernames and one that I found (coupled) which make might for a great name for a dating site.