Mac OS X only: Free, open-source application TuneConnect controls iTunes playback on any computer in your house from any other computer through an attractive interface focused on album art.
In fact, as weblog MacApper points out, it's easy to think of this as a desktop version of the Remote iPhone application. The app requires you to install a preference pane that acts as the server (you'd only need to install this on the computer(s) you wanted to control), meaning the download comes with both the preference pane and the main application—which you'll use on any computer from which you want to control the server.
TuneConnect loads libraries from server computers quickly, and control is snappy. Best of all, the application is free and open source. Unfortunately it's Mac only.