Identify is a small, experimental Firefox extension that quickly analyzes a contact's user name to pull up biographical information, and grab links to their profiles on other social services. For instance, if I were to use it on Rafe Needleman's Twitter profile it would be able to tell me what other services he's using, like FriendFeed, Facebook, Last.fm, and more. The same thing would happen if I checked from his profile on one of those other sites, too.
At least that's how it's supposed to work. It did better on some of my friends than others. A lot of how well the extension can do depends on the two tools it's using to get the job done: Yahoo Query Language and Google's Social Graph API, the latter of which only discovers connections when they're both public and in its index. Creator Madgex has married the two tools together to quickly figure out where the profiles are and provide links directly to them.
The entire process only takes a few seconds, and works via a keyboard shortcut, Alt+i on PCs and Ctrl+i on Macs. This is easy to remember--unless you're going back and forth between machines.
An alternate to using services like this is to take someone's full user name and run it through a name checking service like the recently-reviewed KnowEm and Namechk. If they've got the same user name across multiple sites, this takes some of the gopher work off your plate.