It sounds way too good to be true, like something Fools dream about when they nod off after a long evening of paying bills and reviewing retirement fund statements. But it's not a fantasy.

A number of credit card companies now offer cards that will put a percentage of each purchase into a savings account. It's a kind of a twist on the traditional cash rebate programs. But instead of getting a check in the mail, the rebate is tucked away into savings.

Some of the cards will contribute to regular savings accounts, some to college savings accounts, and others even offer to fill your brokerage account.

What's the catch?

Let's cover the big caveat first. Even if your credit card is contributing a tiny bit to your net worth, you can do a lot of financial damage by carrying a balance, paying interest charges, and incurring fees. In fact, you can easily spend more than you're saving through the rewards program if you're not careful. That means the effect on your bottom line will be negative.

Also, like anything and everything related to credit cards, it always pays to read the fine print. When credit cards and savings accounts combine, that means you'll have at least twice the legal mumbo-jumbo and account details to wade through. Get out the reading glasses.

You'll want to familiarize yourself with the terms of the credit agreement and compare things like interest rates, annual fees, and fines for late payments. You'll also want to look at the terms of the savings accounts and see whether they fit your needs.

The programs
Enough with the scare tactics. Let's move on to the details. These are some of the cards available and some of the fine print:

  • ShareBuilder Platinum Visa: Offered by J.P. Morgan Chase (NYSE:JPM), users earn a point for every dollar spent on credit card purchases. When you accumulate 2,500 points, you get a $25 deposit into your ShareBuilder brokerage account. There's no annual fee. The late payment fine is $39 for balances of $250 or more.
  • One: This American Express (NYSE:AXP) card deposits 1% of your purchases in a high-yield savings account at American Express Bank. Right now, the interest rate on the savings account is 5%. The $35 annual card membership fee is waived the first year (but presumably not in subsequent years). The card does not charge interest on new purchases when the holder carries a balance.
  • Keep the Change: This Bank of America (NYSE:BAC) program is available to customers who have a checking account, debit card, and savings account with the company. Bank of America will round up each debit card purchase to the nearest dollar and put the extra change into a savings account. For three months, Bank of America will match 100% of the savings. After that, they will match 5% of the savings every year. The regular savings account can incur monthly maintenance fees and excessive withdrawal fees depending on the account balance.
  • Upromise Platinum Select MasterCard: Offered by Citibank (NYSE:C) with Upromise, this card will deposit a percentage of credit card purchases to a 529 college savings plan. You can earn 10% on participating grocery and drugstore items, 2% on gas at ExxonMobil (NYSE:XOM), and 1% on everything else. Card members have to make sure they use a loyalty card or register on the Upromise website to get some of the higher rewards. The default rate on the credit card is 32.24%.
  • Fidelity offers two credit cards with savings rewards. One lets users earn a point for every dollar in credit card purchases. At 5,000 points, a $75 deposit is made to your brokerage, IRA, or other account. The second deposits 2% of credit card purchases to a Fidelity 529 college account. The interest rate for purchases is 15.9%. There's no annual fee.

Foolish bottom line
Even with all these creative new savings options, the best way to build a sizable nest egg may be the old-fashioned way -- spend less than you earn and put the rest in a savings account. (Head on over to the savings center for more help with that.) Setting aside 1% of your credit card purchases may not amount to a whole lot of money and could put you on the slow boat to savings paradise.

On the other hand, every little bit counts. If you're scrupulous about paying off your credit card balance each month, one of these offers could improve your bottom line a little bit. Just don't justify purchasing that new Sony PlayStation by telling yourself, "I'm saving!"

For related Foolishness:

JP Morgan Chase and Bank of America are Income Investor picks. Discover more great dividend-paying stocks with a free 30-day trial.

Fool contributor Mary Dalrymple does not own shares of any stock mentioned in this article, and she welcomes your feedback. The Fool has a disclosure policy.