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. The main TuneConnect application is the one you'll use on any computer from which you want to control the music.
TuneConnect loads libraries from server computers quickly, and playback control is snappy. Best of all, the application is free and open source. Unfortunately it's Mac only.