Hvr.me is a fairly new lightweight plugin for your web browser (currently available for Firefox, IE, and Chrome) that allows you to not just get basic information about various tweeps, but also lists all social networks that a user has set up. Just go to Twitter.com after you've installed the plugin and drag your mouse over any user in your home feed. A small window will pop up with icons representing the various networks which when clicked on, take you immediately to that person's other accounts on those networks.
While there's nothing really groundbreaking about this plugin conceptually, after using it for a bit, I admit it has been nice to not have to leave Twitter.com so that I could then go Google someone to figure out what networks they're on.
Any tool that reduces click paths to connect with others on multiple social sites quickly, is a definite win. My only caveat is that I had a little trouble getting the plugin to activate on the Chrome browser (Mac). It just wouldn't work. However Chrome my install of Chrome has been moody lately so maybe it's just me. Firefox (Mac) was fine.
Check out the video below for a complete step-by-step.