iTunes is a free application for Mac and PC. It plays all your digital music and video. It syncs content to your iPod, iPhone, and Apple TV. And it's an entertainment superstore that stays open 24/7.
iTunes also allows you to bless locally networked computers with Home Sharing privileges, allowing unrestricted access to their music, videos, podcasts, apps, and playlists, which can be copied between computers directly within iTunes. It's a great feature for families or any multicomputer household, and can even be set up to transfer any new iTunes store purchases between all of your computers automatically. Of course, content added to your library using means other than the iTunes store (heaven forbid) is excluded from automatic updates, but can still be transferred manually through Home Sharing.