There are three levels of power protection available to the home computer user. The levels are:
Uninterruptible Power Supplies
While this HOWTO mainly focuses on UPSs, we'll start with some basics about the other two kinds of power filtering to help you understand where UPSes fit in. This is useful even though plummeting UPS prices have made the low-end alternatives less interesting than they used to be.
These are basically a fancy fuse between the source and your hardware; they clamp down spikes, but can't fill in a low voltage level or dropout.
This is a bare minimum level of protection that any piece of expensive electronics should have. Note that this applies to more than just AC power; surge suppressors are available for (and should be used on) phone lines, and RS-232 and parallel connections (for use on long lines; generally not needed if the devices are colocated with the computer and all devices are protected from outside sources). Note also that all devices connected to your computer need to be protected; if you put a surge suppressor on your computer but not your printer, then a zap on the printer may take out the computer, too.
An important fact about surge suppressors is that they need to be replaced if they absorb a large surge. Besides fuses, most suppressors rely on on components called Metal-Oxide Varistors (or MOVs) for spike suppression, which degrade when they take a voltage hit. The problem with cheap suppressors is that they don't tell you when the MOV is cooked, so you can end up with no spike protection and a false sense of security. Better ones have an indicator.
You can buy surge suppressors at any Radio Shack; for better prices, go mail-order through Computer Shopper or some similar magazine. All of these are low-cost devices ($10-50).
These devices filter noise out of AC lines. Noise can degrade your power supply and cause it to fail prematurely. They also protect against short voltage dropouts and include surge suppression.
The Tripp-Lite 1200 I used to have was typical of the better class of line conditioners; a box with a good big soft-iron transformer and a couple of moby capacitors in it and no conductive path between the in and out sides. With one of these, you can laugh at brownouts and electrical storms. A fringe benefit is that if you accidentally pull your plug out of the wall you may find you actually have time to re-connect it before the machine notices (I did this once). But a true UPS is better.
Netter Trey McLendon has good things to say about Zero Surge conditioners. He says: "Our systems at work [...] have been protected for 2.5 years now through many a violent storm...one strike knocked [out] the MOV-type suppressors on a Mac dealer's training setup across the street from us. The Zero Surge just sort of buzzed when the surge came in, with no interruption whatsoever. The basic principle is this: ZS units slow down the surge with a network of passive elements and then sends it back out the neutral line, which is tied to ground outside at the box by code. MOV units shunt the surge to ground at the computer, where it leaps across serial ports, network connections, etc. doing its deadly work."
Price vary widely, from $40-400, depending on the power rating and capabilities of the device. Mail-order from a reputable supply house is your best bet. Line conditioners typically don't need to be replaced after a surge; check to see if yours includes MOVs.
The remainder of this document will focus on UPes. A UPS does three things for you. First, it filters the power your machine sees, smoothing out spikes and voltage fluctuations that can stress or even damage your electronics. Secondly, it provides a certain amount of dwell time in the event your power goes out entirely — this can often get you through brownouts and short blackouts. Third, when the UPS is about to run out of power it can arrange a graceful shutdown of your computer so that no unpleasant things happen to your disk filesystems. While the risks of unexpected shutdown are much lessened in these days of journalling filesystems like Linux's EXT3 or JFS from what they once were, ensuring a clean shutdown is still a valuable contribution to any system administrator's peace of mind.