MacOS System 7
Apple Computer, Inc. -- 1990
The Finder is the program that runs the Mac OS desktop (like the Windows desktop) and manage files (like the Windows Explorer). During the development of System 7, Bill was asked to help the Finder team significantly enhance the Finder. Some of his suggestions were implemented immediately. Some, like being able to resize column widths in directory listings, or moving the columns into different orders, didn't get in until eight years later, in Mac OS 8.5 (and they'd probably forgotten all about Bill by that time). Here are two of Bill's ideas that were implemented in System 7.
Prior to System 7, you could cycle through the open application programs by repeatedly clicking the icon at the right end of the menubar:
Bill suggested making this icon into a pull-down menu so that users could go immediately to any desired application:
In-line Folder Expanson
The Finder team wanted to add a new "mode" where, depending on a folder-specific setting, double-clicking on a folder icon would either: (a) open a new window, or (b) show its contents as an indented list in the same window. But: You wouldn't be able to tell by looking at a folder icon which mode it was in.
Bill's suggestion was to add a row of "expansion" controls down the left side of the window. This would: (a) make the new functionality obvious, (b) avoid adding a "hidden" mode, (c) eliminate the need for users to set folder-by-folder behavior preferences, and (d) let double-clicking open a new window as it always had:
Initially Bill proposed using empty and filled circles. But before shipping these were changed to the now-famous rotating triangles: