Currently in closed beta, Brizzly is a Twitter and Facebook client built by many of the same folks who built Google Reader. Its maker, Thing Labs, is headed up by Jason Shellen -- a former veteran of Blogger, the startup Twitter's Evan Williams sold to Google in 2003.
One of the cool features of Brizzly since it went into private beta this summer was Groups, which worked essentially like a precursor to Twitter's own Lists. You could sort followers into groups in much the same way, and the way Brizzly chose to handle the transition from Groups to Lists is spot on.
Brizzly will display the lists you've already created on Twitter and give you all the tools you need to create, display, sync and navigate them. The Groups you've already created in Brizzly will be migrated back in the other direction to Twitter as well -- but as private lists by default, in case you've made Groups you might not want to share publicly (you can of course change the setting to public if you wish).