Do you want to play classic Sierra adventure games right from your browser? Visit Sarien.net.
This site ported a number of classic Sierra adventures to Adobe Flash and made them playable online. You can actually see other players play the game, but everyone's game has its own "instance", meaning you can only interact with your own copy of the game. Commands you type to the parser appear above your head, so you can use that to type messages to other players.
2009-05-10 edit: King's Quest added. 2009-05-31 edit: King's Quest II and III added. 2009-10-20 edit: Gold Rush! added.