The Theory of Leech Computing
Phil Frisbie, Jr. writes "I am defining Leech Computing as 'a program running on a client computer without user knowledge that can process data and report back the results, but otherwise does not effect the usability of the client computer and makes no changes to the client'. Leech Computing, Part 1 covers basic theory."
Spyware seems to fit this definition as a less-appreciated form of leech computing.
I'm not afraid of falling, it's the sudden stop at the end that frightens me.
There's also a good page quickly discussing Villain-to-Victim computing. The point is to use correctly configured machines to do things they were not intended to.