Neko is a cross-platform open-source animated cursor-chasing cat screenmate application.
This is a complete rewrite from scratch in JavaScript, based on a study of existing versions on various platforms.
It's an evolving work in progress.
<script src="neko.min.js"></script>
<script>
createNeko()
</script>
First created in the late 1980's for the computer NEC PC-9801, this adorable program has been ported to Macintosh, X Windows, Amiga, Windows 3.1 & 95, IBM OS/2, iPhone, Android, and even Arduino.
See also Wikipedia: Neko (computer program).