Domain Pigeon helps you find unclaimed URLs
Want to find an open domain? Check out Domain Pigeon. This service lets you search and sort through open names, and shows you what others have been clicking on.
There are far too many domain search and purchasing tools, but I haven't found one that does it like Domain Pigeon. The service tracks domains that are still available and puts them in an index that can be searched and sorted in all manner of ways.
Domain Pigeon's secret sauce, however, is that it shows you what other users have been looking at, right down to how many have clicked on any specific domain name. Domains that have gotten more attention darken in color, with the heavy hitters bubbling up to the top.
To encourage registration (which costs a one-time fee of $15), users who sign-up can view and search from a larger catalog of domain names. As of me writing this it's a difference of an extra 5,000 or so domains once you register.
While the site doesn't actually sell domains--something you have to do with a registration service--it can be a good source of inspiration for people who have not found a good name for their service. Of course, if you're trying to be secretive about that process, you're probably better off using something that won't be showing your search queries to a gaggle of other users.
See also:
Bust A Name: Best domain name helper ever
Eleven steps to buying a domain name that doesn't suck
What's in a domain name?