robotfindskitten is a "Zensimulation", originally written by Leonard Richardson for DOS. It is a free video game with an ASCII interface in which the user (playing the eponymous robot and represented by a number sign "#") must find kitten (represented by a random character) on a field of other random characters. Walking up to items allows robot to identify them as either kitten, or any of a variety of whimsical, strange or simply random Non-kitten Items (NKIs). It is not possible to lose (though there is a patch that adds a 1 in 10 probability of the NKI killing robot).
The original robotfindskitten program was the sole entrant to a contest in 1997 at the now-defunct webzine Nerth Pork — the object: create a depiction of "robotfindskitten". (The "robotfindskitten" concept was originally created by Jacob Berendes, but the only submission he received depicted kittens meeting an untimely end at the hands of malevolent robots.)
When the author rewrote the program for Linux in 1999, it gained popularity and now has its own website and mailing lists. Since then, it has been ported to and/or implemented on over 30 platforms, including POSIX, the Dreamcast, Palm OS, TI 99/4A, the Z-machine, the Sony PSP, Android, and many more. Graphical versions, such as an OpenGL version with # emblazoned on an otherwise featureless cube, also exist. Remakes of it are also used as programming tutorials, such as for Gambas.