From the mid 90's to early 2000's I wrote plug-ins for Macromedia Director under the name Andrade Arts.
Xtras were basically DLLs for Windows, XCMDs for MacOS 8/9, and dyLibs for MacOS X.
Some of my most popular Xtras were Enhancer, Resolution, and Move Cursor.
The Xtas also worked in the browser. They were available by signed download through the shockwave plugin. The xtra brought things like true full screen in desktop and browser, mouse lock, and game controller functionality at a time when these capabilities were not readily available. All of this worked seamlessly across multiple platforms; Win32, MacOS 8-9 (Carbon), and Mac OSX.
My Xtras were used on some of the most popular web game portals in the world. Sites like MiniClip, Nick.com, 3D Groove and others offered a richer game experience using the xtras.