Apple's iTunes 9 introduced a feature called Home Sharing that makes it easy to add music to your laptop and share music between as many as five computers on one network. You can use Home Sharing to stream music on your computer from another computer on your network, add music from your iTunes library to another networked computer or add new music to your laptop from your networked desktop. All it takes is an iTunes account and a local area network.
Open iTunes on your laptop, and click on "Home Sharing" in the left pane.
Log into your iTunes account and click "Create Home Share."
Open iTunes on all of the other networked computers in your home, and log into the same iTunes account.
Go back to your laptop, and look for the networked computers' iTunes libraries in the left pane of iTunes.
Click on "Settings" and select your desired categories, or drag and drop selected songs into your own library on the laptop. Select "Items not in library" to move songs from the chosen iTunes library to your laptop's iTunes library.