The old Infocom text adventure game The Hitchhiker’s Guide to Galaxy has been converted into a Java applet and put online!
The only downside is that since it’s a Java applet, you can’t save and restore as you play. Whenever you lose the game, you have to start at the beginning. There will be versions that run under Windows and Mac released soon, but they’re not available yet.
(I remember having a helluva time trying to figure out how to get off the Heart of Gold.)
[Thanks to Starjewel for pointing this one out.]