Just about every network administrator understands the importance of using conditioned power and a battery-backup UPS to keep the network up in case of power outage. This is particularly important if you use Ethernet-based VoIP phones and Power-over-Ethernet because your phones will be completely disconnected in the event of a blackout and that's something your users are probably not expecting, since regular office phones typically still work when the power goes off. Most administrators also realize that the "power supply" component inside a switch or router is one of the most susceptible to failure.
To prevent the loss of a power supply from becoming a "single point of failure" and taking out the network, chassis-based routers and switches often support two or three power supplies, but most organizations deploy smaller appliance-like routers and switches that don't have big, hot-swappable power supplies.
If you use smaller routers and switches, and are concerned about having a highly available network, you probably still have some options. First, your device may actually have a second power socket, but it won't be the standard 3-prong electrical cable. It probably looks like two long rows of large, white plastic holes, although it varies by manufacturer. To take advantage of this, you would purchase a separate power unit like the Cisco "RPS" system, and run cables from this external power supply to several of your routers or switches. In the event your internal power supply fails, power would fail over to the external box fast enough that your network device shouldn't reboot.
If your router or switch doesn't have one of these connectors, you're not out of luck just yet. A second option is that some manufacturers will let you crack the chassis open and replace the regular AC power supply with a special power unit that connects to the external power supply. (Routers like Cisco's 2500 and 2600 series fall into this category.) The advantage of this is that the failure-prone parts are in the external supply, which should be redundant. Although the connection in your network device still represents a single point of failure, it's much less likely to cause you trouble than a regular power supply.
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.