Network interfaces in most wired or wireless computing devices are greatly under-utilized, squandering energy while waiting for new packets to arrive. In this paper, we propose simple opportunistic algorithms to determine when to power down idle network interfaces in order to save much of that wasted energy. We also develop an analytic model to predict the amount of energy saved and the packet delay. The algorithms are compared to the best known opportunistic method, and it is shown that they provide significantly greater power savings without inflicting noticeable delays to Internet traffic.