The greatest legalized scam in the history of capitalism for those who use it properly. The benefits of free credit are responsible for me making between ten and twenty dollars of free interest every month, just by scheduling my payments right.
Credit cards are the most common way to take advantage of free credit. I have exactly one, and because it's a Visa, it's the only one I ever need. I make all my large or online purchases using it. At the end of the month they send me a bill. The bill says that I have something like three or four weeks to pay it, so I take advantage of the added features of online banking and schedule a payment to be made just a few days before it's due. Result: the money I would have spent right away if I'd used cash or a check has been happily accruing interest for nearly two months. Voila, free money.
But even better than credit cards is the gift of free financing. You see this all the time on television: "Buy now and make no payments for six months!" or words to that effect. When I bought my fiancee her engagement ring, this is exactly what was offered. I had enough money to pay off the ring in cash right away, but I'll take free money when I'm offered it. Do a little math, calculate how much it would take each month to pay the ring off in exactly six installments, and pay that on time every month. So now I have nearly two months' salary sitting in my account earning interest for me. Voila again.
This technique involves a little bit of strategic planning, of course. I use Quicken personal finance software to keep track of every credit card purchase as I make it; that way, I record those purchases as money already spent, even if my bank doesn't. I schedule my credit card and engagement ring payments at the latest possible date. (Even if you don't use online banking, your credit card company may have a Web site allowing you to schedule electronic payments.) I use an interest-bearing checking account as well as a higher-interest savings account. I use a credit card issued by a reputable bank, so I don't have to deal with sneaky tactics like getting my credit card bill bare days before it's due. And I make it a policy never to spend money I don't already have, so there's zero chance of me going into credit card debt.
Granted, ten to twenty dollars of free interest every month isn't going to make me a millionaire. But any amount of free money is a good thing. Besides, thanks to compound interest (the second-greatest legalized scam in the history of capitalism), those dollars will eventually accumulate high enough to buy me a new stereo or sofabed, all courtesy of my friendly neighborhood bank.