The Hitchhiker's Guide to the Galaxy

Confession: I never played any of the original versions, instead coming on board with the BBC Radio 4 20th Anniversary Edition, which is in many ways kinder to the player; it came complete with the game itself, but also some simple but pleasing graphics, interesting game design notes, and all the Douglas Adams hints. That page is still here, although the game is currently hosted here. (Apparently this is the 30th Anniversary Edition, which is like the old one but with a shinier interface and, erm, the ability to tweet from in-game. Perhaps just as well, if the brush-up allows the game to be around for another ten years.)

Anyhow, it's gloriously implemented and perfectly atmospheric. Definitely worth a try if you're at all a fan of the radio series/books/movie/stageshow/whathaveyou.