Apple's iTunes application is an excellent piece of software. As of November 2005, iTunes 6 provides a means to play audio (in formats as diverse as WAV, MP3, and AAC), video (any QuickTime-compatible format), download podcasts, and purchase songs and videos via the iTunes Music Store, or iTMS, currently the most successful online (legal) music store.
iTunes can be downloaded from Apple's website, http://www.apple.com, for both Mac OS X and for Microsoft Windows (2000 or above), as well as being bundled with every new Macintosh. It also forms a part of Apple's iLife suite of 'lifestyle' products, although it is the only member of the suite to have been ported to another platform. Older versions of iTunes which will run on Mac OS 9 are still available from Apple's support website, and with a little tinkering, iTunes 1.1 can be convinced to install and run on Mac OS 8.6.
When using iTunes, one imports songs and videos into a central library, which consists of a database within the current user's directory (on Mac OS X, in ~/Music). Users can play music directly from the library, or separate playlists can be made, or you can simply play randomly-selected tracks in the 'Party Shuffle' mode. This allows all users to have their own libraries of music, but if one wants to share iTunes libraries between users, there is no obvious way to do so. Thankfully, it is possible, at least under OS X.
First, make sure you're logged in as an administrator, and close iTunes, making sure it is not running. (Look under its icon in the Dock: if it has a triangle (or glowing dot) beneath, it's still running in the background. A quick Command+Q from within iTunes will ensure it's closed.) Now, move the 'iTunes' folder from the user whose library you want to share, into a directory everyone can access. (Their library will be in /Users/username/Music/iTunes, so it's this folder we want to move - put it somewhere like /Users/Shared)
We need to change the permissions of this folder, so that everybody can read and write to it. Either use the terminal, or Get Info on the folder, and under 'Ownership', set everything to Read & Write. Click "Apply to enclosed items", which will set these permissions to the folder's contents, then close the Info window.
Back in the Music folder in the user's Home, create an alias to the shared iTunes folder, renaming it back to simply 'iTunes'. When iTunes runs, it will then follow this alias to the real location of the library. All you need to do now, is to log onto every account you want to share the library with, delete their iTunes folder, and create the alias to the shared folder.