ScummVM is a great open source project for running old quest games like Simon The Sorcerer, King’s Quest, Maniac Mansion, Day Of The Tentacle, and… I can go on with the list forever. Basically, this project was created to run old Sierra games, but since then has been developed to do a lot more than that.
Note that ScummVM is only the runtime engine for the games. To play an actual game you have to find the files on the Internet (this is due to legal issues). It’s not that hard, eh hem eMule eh hem… excuse me, I had to cough ;-)