And it's not just the planet you're saving, but money as well. Compact fluorescents use 66% less energy to provide the same amount of light and last about ten times longer than a standard light bulb. For each bulb you replace, you'll save (at current electric rates) about $30 in energy costs over the life of the bulb.
We've gradually been replacing our bulbs with CF's as they burn out. We're starting to notice the savings on our electric bill. The bulbs cost a little more than traditional bulbs, but they go on sale fairly regularly. You can buy them pretty much anywhere you buy regular lightbulbs.
Click here for more information on compact fluorescents from the Energy Star website.