iTunes Remote Control lets you control another machine's iTunes playback over a network.

iTRC is a Universal application, so it will run on (and between) Intel and PowerPC Macs seamlessly. It is free and open source.

Setup is simple, just enable Remote Apple Events (System Preferences->Sharing) on the computer that is running iTunes. Make sure there's a network name entered in the 'Computer Name' field.

Open iTRC on the computer you want to use as the remote control, and type in the network name of the iTunes server. Authenticate with the username and password of the currently logged-in user of the iTunes server computer, and you're off to the races.

It provides support for track changing, playlist and EQ selection, and control of Shuffle and Repeat functions. It also includes Growl support. Works (for me) with iTunes 6 and 7.

Gotchas:

-Make sure to add ".local" to the end of the iTunes server's network name.
-Do not use the Keychain option to store your password, the program will crash on launch.
-Make sure the username is correct, as it defaults to your current machine's username instead of the remote one.