PlayerPal will let you control iTunes or Windows Media Player from any device with a browser. Your PDA, laptop, playstation portable etc.

NetRemote and Girder could give you similar control, but PlayerPal looks like a pretty interesting and user friendly solution.

It will work as long as you have a browser on the device you want to use as a remote. No client needed. I haven’t had time to try this one out yet, so I’ll have to get back to it.