Macintosh Guide
Apple Guide was Apple Computer's online help and documentation system, added to the Mac OS in System 7.5. In addition to hypertext, indexing and searching of the text, Apple Guide also offered an extremely powerful system for teaching users how to accomplish tasks in an interactive manner. However, the process of creating guides using the system was so complex that few developers took advantage of its power. Apple Guide was abandoned when Apple moved to Mac OS X, who's help system is based on HTML.
Key to Apple Guide's power was the use of the AppleEvent Object Model (AEOM), allowing the system to examine the state of the application as it ran, and change the help in response. For instance, if the user had already completed half of an operation and needed help to complete it, Apple Guide could "see" where they were, and forward to the proper section of the documentation. Additionally AEOM allowed Apple Guide to drive the interface, completing tasks for the user if they clicked on the "Do it for me" buttons (or hypertext).
Perhaps the most striking feature of the system was support for Coaching. Using the AEOM, AppleGuide could find UI elements on the screen, and circle them using a "red marker" effect to draw the user's eye to it. This feature remains unique to this day, and although it was somewhat slow (deliberately), it was certainly one of the most useful features of any help system.
Apple Guide was also somewhat integrated with Balloon Help, optionally adding hypertext to balloons that would open the right portion of the documentation based on what object the user was currently pointing at with the mouse.
Since the full power of Apple Guide was based on having easy access to the internal workings of the applications via AEOM, it's not terribly surprising that so few applications supported it; adding support for AEOM was itself an extremely complex task that few programs took advantage of. While most Apple software was supported with complete Apple Guide documentation, very few 3rd parties used it for anything other than a hypertext viewer, its promise left unused.
One particularily annoying "feature" of Apple Guide is that it was a separate application from the one it was providing help for. When the application was closed, the guides would remain open.
Even when it was available, many developers used HTML for documentation anyway, allowing them to place the identical source on both the application installer and their own web page. Moving to a completely HTML-based system for Mac OS X was a rather practical decision.