Historically, of course, an album was a slab of vinyl twelve inches in diameter. A vinyl album is actually capable of holding almost as much as a CD (there are records clocking in at just shy of 70 minutes), length on an album comes at the expense of sonic fidelity, and resistance to getting scratched. The majority of albums released during the vinyl era clocked in at about 40 minutes.
There are exceptions, though. Some albums are really terribly short, perhaps because the songs on them are short, or perhaps because the artist felt the work was complete as it stood, or perhaps because the artist simply ran out of creative inspiration - or studio time. Whatever the reasons behind them, the following albums are all incredibly short - shorter than some EPs or even some singles, in fact.