In honour of the 30th anniversary of the brilliant (and incredibly frustrating) Hitchhiker's Guide to the Galaxy text adventure game, BBC Radio 4 Extra have recreated the game in an online edition. I remember playing this for endless hours, with my Peril Sensitive Sunglasses perched on my forehead, repeatedly typing "look." The R4 version allows for saved games, so you can come back to it. You can also play the 20th anniversary edition.
(Thanks, Fipi Lele!)