Why’s “Try Ruby” (Web Version of irb) Back Online
Try Ruby was a Web site by Why The Lucky Stiff that provided a Web-based version of irb (the interactive ruby prompt) and a 15 minute tutorial for people to learn and play with Ruby. With Why's disappearance, however, the site went down and an invaluable Ruby community resource was lost.
Luckily, Andrew McElroy has made a great effort in getting Try Ruby back online. It's not precisely the same, but it's as close as you're going to get for now (one key difference is that the irb process is not persistent - instead the history is re-run on each new line). There's even the 15 minute introductory Ruby tutorial! An extra bonus is that Andrew has released all of the code he has for Try Ruby in a Github repository. Why's code was never open sourced but Andrew has done a good job in starting to rebuild the backend.
Note that this is not a Why-approved project (yet). To the best of my knowledge there is still no further information on Why's disappearance.