For awhile now I've had an Itunes compatible music server in the form of open-source Firefly (previously called mt-daap) on Ubuntu (Gutsy and Hardy). It takes a little setting up, but you can grab firefly from Ubuntu's repositories make sure you have Apache running for web configuration. There's a nice tutorial here, and plenty of others with a little Googling. Update 05/11/81: the link is dead, try this tutorial (which is based on Ubuntu)
On either the host machine or another Linux PC on the network, you can use Amarok (daap shares should be available under 'Devices' along the left hand side, enter password if you set one up.) or you could use Rhythmbox or Banshee if you prefer.
On Windows you could use Itunes of course if you don't mind the bloat, but there are some alternatives. I just discovered Get It Together (GIT) , an open-source Itunes client for Windows, Mac or Linux. It uses a java based mp3 player internally, but can also use Quicktime, which I would only use for playing AAC files. It can also download tracks from the server. I also look forward to Amarok being available on Windows , which should be the ultimate Itunes replacement!