Adventure Game Studio: Allows creates graphical point-and-click adventure games.
Adventure Maker: Program to make first-person adventure games, such as Myst or Riven. Downloads and screenshots available.
Hungry Software: SLUDGE: Standing for "Scripting Language for Unhindered Development of a Gaming Environment", SLUDGE is designed specifically for adventure game creation and is the language used in Out Of Order.
Insanity Games: Makers of AdvEngine, a graphical point and click adventure game engine in development for games akin to Monkey Island 3.
InterAx Game Engine: Point and click editor for creating simple graphical adventures. Includes screenshots, sample games and editor downloads, mailing list, FAQ, and purchase information.
MAD -- adventure game engine: A free graphical adventure game engine, it is completely script-driven and highly customizable. Mad features an archive system, animated sprites, mp3 playback, and path finding. A beta is currently available.
ScummVM: An implementation of LucasArts S.C.U.M.M. interpreter, used in games such as Monkey Island, Indiana Jones, Day Of The Tentacle and Sam and Max. Includes FAQ, screenshots, documentation and downloads.
TROIACS - Text Rooms and Objects Interactive Adventure Creation System: A sierra/lucas style adventure game engine in development.
Twilight Software AGE Developer Pages: The engine allows for the creation and delivery of commercial-grade first-person adventure games and other multimedia.