I used a few times, it was one of the first "toy" programs I found. There is also a linux package (oneko) that does the same thing. Other "toy" programs I had was "Bart.exe" (Bart Simpson appeared behind the window borders) and another one (also had a linxu version) that hid cockroaches behind the windows... so every time you minimized a window you saw some cockroaches running to hide behind another.
BTW, in a cat-software-related thought, I miss the cat assistant in Office 2000. I mean the drawn cat, not the paper cat (ooooh... I get the word play) found in Office 97. I configured it to not give suggestions and left it "playing" where it didn't annoy me on screen, so it was sleeping most time and only moving when saving, printing and the like. I feel it relaxing.
Maybe it was because I had an orange tabbed cat that spent most of my ZX Spectrum years sleeping above my TV (CRT screens... most cats loved them), so I feel "alone" without it.
I have traveled across the universe and through the years to find Her.
Sometimes going all the way is just a start...
I'm selling some stuff!