Alcatraz Island is one of those tourist traps that isn't a trap at all. It is that rare thing: a justifiably famous tourist attraction that lives up to its reputation and exceeds it. When I lived in San Francisco, I relished the chance to take out-of-towners there and re-visit it myself.
The site itself is exceptionally beautiful, a rugged wilderness island in the middle of the San Francisco Bay with unparalleled views of the city. The cellhouse audio tour — a self-guided tour featuring the narration of former inmates and guards — is brilliantly produced (I actually bought a copy on cassette years ago and listened to it at home). And the additional museum materials, including a moving film on the history of the Indian occupation, are also superb.
The Alcatraz website does a good job of conveying much of this, but you really have to go to experience it.
(Thanks, Ranger Craig!)