Open Quest is an open source adventure game that was created to help people create an adventure game, by examining the code from this example game.
It was created by Michael Sheail in October 2007, using the most popular adventure game development engine, Adventure Game Studio.
He released the code and the assets freely, giving everyone permission to use and extend the game.
Due to the open nature of the release, it was then used by the team behind ScummC, an open source compiler to make games using the LucasArts SCUMM engine. The ScummC team also released the source code of their version openly with permission to use and extend it.
This site isn't associated with Michael Sheail or the ScummC team. It was created by Jennifer McMurray on August 10, 2013 to support and share this excellent learning tool, that's a fun game too!
Open Quest for AGS with source code
Open Quest for ScummC with source code (Requires ScummVM to play)