All things command-line wizardry.
You'll often need to run a command over and over again for some reason (mine right now is: I have to check if a file exists). There are obviously better options - but doing things through command-line ninjutsu is always funnier. So, one-line infinite loops in Shell (both sh and Bash):
while true; do echo "Hello!" && sleep 1 done
This will run
echo "Hello!" forever, "sleeping" for one second between each execution.