Stateful netfilter/iptables IPv4 packet-filtering firewall bash scripts.
There are two scripts, one to create a simple router, and one for a standalone server's firewall.
More than anything, these are distillations of various firewall techniques. Not the easiest-to-use setup, but separate tables make accounting and monitoring theoretically easier.
Invented in the Yukon, hence the "Klondike" name.