Here's a rather old (and probably outdated) look at how one could simulate digital logic circuits with OpenTTD. Includes the fastest four-bit ripple-carry adder ever: takes about two months (of in-game time) for the carry information to propagate.

From the 2005s, an outdated nano-HOWTO on how to build a port knocking system to reduce the amount of SSH password guessing attempts, with simple iptables rules and no extra daemons.

Document here.