Wade Billings pointed out knockd to me. This little program lets you set up secret port knocks and then take action when you see them. So, for example, you could set up a server with all ports locked down and then open up SSH just when you needed it by knocking on selected ports in a specific order. The ports you knock on don't have to be open since the daemon listens to the ethernet interface. If you're really paranoid and only a few people need to access the server, then this might be for you.