iTunes 9 takes sharing between computers one step further. You could always listen to tracks that were in iTunes on another computer, but you needed to have the computer on all the time. Now you can easily copy the music from other computers in your house to your library.
Perform these steps on all computers you wish to share music with.
2. Select File > Home Sharing > Turn On Home Sharing.
3. If prompted for a username and password, enter your Apple Store credentials.
4. Repeat steps 1 through 3 for any other computers you wish to share music between. Ensure all computers are connected to the same network.
For each computer you wish to share a library from, perform steps 5 through 7.
5. Click Edit > Preferences.
6. Select the Sharing tab.
7. Check Share my library on my local network to share your entire library. Optionally, you could select Share selected playlists, then check individual playlists if you wish to limit sharing. You can also require a password for access to the library allow play counts to be updated through sharing usage.
8. You will now have a section that says Shared on the left sidebar. It will list all libraries that are shared over the network.