Once upon a time I wrote a java applet, which looked like it was showing an animation, but was secretly calculating digits of PI. Over a period of a few months

Once upon a time I wrote a java applet, which looked like it was showing an animation, but was secretly calculating digits of PI. Over a period of a few months I got quite a decent result from people who'd just happened to visit my website. Not quite as malicious as your story but equally nefarious.