Instead of visiting your friend's house to rip all of her CDs one by one, you can save yourself the trouble with Mojo — it's a download that gives you convenient access to your friends' iTunes libraries. Now, you can't download Mojo and just grab everyone's music willy-nilly — the friends need to be on your network at the same time, and they need to have downloaded Mojo as well.
After you've both downloaded Mojo, you get your friend's username and add each other. When you open iTunes, the compatible library will show up in the left margin, and as you browse through his or her library, there's a download option next to it you can select when you see something you like.
It's easier and more efficient than sending songs and albums via IM or email, and it's available for both Macs and Windows. Have more questions? Check out the Lifehacker post for more details!