Adobe Air: There's plenty of standlone Twitter clients, but Facebook options are less common. SocialVisor combines both in a single Adobe AIR client to let you keep an eye on your social networks without too much productivity disruption.
SocialVisor runs in a narrow horizontal strip, scrolling updates across to the left and offering buttons for retweeting, direct messages, and replying to comments. As with most clients, you'll either think "that's perfect" or "no way can I work like that" -- it's undoubtedly slower to read a bunch of comments in this mode than on a normal screen, and it's a setup that's more appealing in a multi-monitor environment than on a notebook.
SocialVisor is currently in beta, and there are a few rough edges showing -- I suffered random exits and an inability to retain my Twitter details, and the claimed conversation threading features didn't seem to be working. SocialVisor is a free application for Adobe AIR.