Watcher puts eyeballs in your menu bar that keep an eye on your mouse cursor. It's not at all useful, but its fun, and its compatible with every version of the Mac OS, from 7.0 through 8.1.
But best of all, its free!
Installation: Drag Watcher to your Extensions folder inside your System Folder and restart your computer.
There is a space before Watcher's name (Watcher). This is designed to make it load before the other extensions, so that the eyeballs will be the leftmost item on the right side of the menu. You can remove the space, or change Watcher's load order, but the eyes may be sandwiched between other iconic menus and the application menu.
Notes: Watcher requires color QuickDraw. This means that it may not work on a PowerBook 100, an SE/30, or any other classic Macintosh.