Fluorescent light bulbs have been around for a long time, but there are still a lot of households that don't use them. They should.
Let's do a back-of-the-envelope calculation about how much one of these bulbs is worth.
Assume that we replace a traditional 100-watt traditional incandescent bulb with a 23-watt fluorescent bulb. The traditional bulb costs $0.50 and lasts for 2,000 hours of light, whereas the fluorescent bulb costs $X and gives 10,000 hours of light. Electricity in Applied-Genius land costs $0.135 per kilowatt-hour these days (twice the 1999 rate!) Finally, assume for the sake of argument that you're flat broke and would have to buy the bulbs on your credit card, paying 20% APR for the privilege. Solving for the price $X that exactly equalizes the cost of doing it the old incandescent-bulb way vs. the new fluorescent-bulb way gives us the Net Present Value of buying an incandescent bulb.
The Net Present Value of buying a fluorescent bulb under these assumptions is -- ready? -- a little over $72. For each bulb. If your money's worth only 8%, the NPV rises to $90+. For each bulb.
At Amazon, these 100-watt replacements are $17 for a 4-pack. At these prices, your NPV exceeds your cost for an gain of $271 per 4-pack. In other words, buying a houseful (20 bulbs) is like winning $1355 in the lottery (and you don't even need to pay taxes on it!) And that's ASSUMING that you need to pay the outrageous 20% interest on your card.
Another way to look at the attractiveness of this investments is the Internal Rate of Return (IRR) method. A $10 bulb returns 172% annually on your investment. A $6 bulb returns 232%; a $4.25 bulb returns 248%, and a $1 bulb returns a whopping 555%. Note that these bulbs are frequently available in dollar stores, as well as at Big Lots and other discount retailers; brand-name GE bulbs are available at Sam's Club for approximately $3 each. (Canadian dollar stores even offer entry-level Sunbeam fluorescent bulbs for $1 Canadian, and their electricity rates are higher than in the USA.)
Note that these are far, far better returns than Warren Buffett has gotten at Berkshire Hathaway, and basically risk-free (and tax-free) to boot. I suspect that Warren, a legendarily sensible sort of fellow, knows about fluorescent lights, and may even have a few in his Omaha childhood home; he'd undoubtedly invest billions in 248%-return risk- and tax-free ventures if they were made available.
(The Applied Genius Bulb Spreadsheet allows you to do these calculations for your situation, including your local electricity rates and the number of hours per day that you use the light.)
Just to beat the already-dead horse to death and beyond, there are a few additional and important (but difficult-to-quantify) benefits as well.
(a) If you air-condition your home, you should realize that you're paying close to twice as much, because most of the energy used by a traditional incandescent bulb is thrown off as heat. You pay twice for that energy: first when it's emitted from the bulb, and again when your air conditioner spits it out of your house.
(b) Fluorescent lights don't require changing nearly as often. You'll spend less time on a ladder changing hard-to-reach bulbs, as well as less time at the store buying them. (Interestingly enough, it's this maintenance-cost savings which drove adoption of fluorescents in commercial buildings, not the power savings.)
(c) Saving electric power means slower increases for future electric power prices. (OK, you're sharing this benefit with everyone in the world, so the direct effect on your own electric bill of you choosing fluorescents is really, really small. But you can feel good about it, knowing that you're in harmony with the first formulation of Kant's Categorical Imperative, which is often inaccurately equated with the Golden Rule but can be better paraphrased as "Do only that which you wish everyone would do.")
(d) Last but not least, you can brag to your friends about how smart you are. (But you should, eventually, 'fess up and tell 'em where you read about the business case for fluorescents.)
Get thee to the bulbery!