This contains all music, movies, podcasts, and TV shows.
This works on up to 5 computers and requires that they sign into i Tunes using the same i Tunes account.
To consolidate libraries, turn on Home Sharing on all the computers you want to merge and then drag and drop the files to the computer that will store the merged library.
Plug the hard drive into the computer that will store the new i Tunes library.
Find the i Tunes folder on the external hard drive, and the i Tunes Music folder inside it.
Unlike the external hard drive method above, these programs let you retain star ratings, play counts, playlists, etc. (If you don't, I'd recommend starting before a hard drive failure makes you sorry you didn't.