A screenshot from King's Quest 4 AGI
AGI (Adventure Game Interpreter) is the adventure game engine used by Sierra on-line to create some of their early games. With the release of King's Quest 1 in the early 80's, it introduced the gaming world to the concept of a 3D graphical adventure game, where the player could move a character around the screen, behind, in front of and over objects. Other commands could be typed in, just like a text adventure. A number of other games that used AGI were subsequently released, such as Space Quest 1 & 2, Police Quest 1 and Leisure Suit Larry 1 to name a few.
The graphics were rather blocky, using a 160x200 resolution, which I believe is because of the graphics mode used by the IBM PCjr, for which the interpreter was originally written. AGI used the internal speaker for sound on PCs, but on pretty much every other system (Mac, Amiga, Tandy, PCjr) it was capable of doing 3 voices.
Since late 1996 a number of people around the world have been working together to figure out the data formats used by AGI, with the intention of being able to view and edit the data. It has taken a while, but we are now at the point where we can do this, and even create our own adventure games using the system.