Gift cards have surpassed all other categories as the most popular gift, according to the National Retail Federation.

Here's a news flash, however: If this is the extent of your gift card experience, you have not tapped into the true money-saving potential of gift cards.

This is a secret that not many people know about. Gift cards can be purchased at less than face value and then used to buy gifts. This simple twist on Christmas will save you 10% to 25% on top of all the insane Black Friday deals, free shipping coupons and Christmas sales. (By the way, Free Shipping Day is Dec. 17 this year.)

Here's how to make gift cards part of your holiday savings strategy, in case you don't have a Christmas budget. (I'll also show you some of the best gift card deals you can find online. But remember: Many of the gift cards sold online are for e-cards, which are sometimes limited to online shopping. In other words, you don't get a physical card.)

The Top Online Gift Card Sellers

First, start by deciding which online gift card seller you feel comfortable working with. There are several prominent ones from which to choose.

PlasticJungle

The site guarantees all transactions, so you don't have to worry about fraud. You are essentially buying gift cards that others have sold to Plastic Jungle. Expect discounts of 2% to 25% off the face value of gift cards to Walmart, Sam's Club and more. Sign up for site alerts for the gift cards you want. Plastic Jungle lets you search by the size of the discount and has the best user interface, in my opinion. When I compared 11 stores, Plastic Jungle had the biggest discounts on seven stores, although the overall selection was not as good as GiftCardRescue.com.

GiftCardRescue.com

You can buy, sell or exchange gift cards on this site. All transactions are 100% guaranteed, and shipping on discount gift cards is free. I saw discounts of 3% to 25% at mainstream retailers, like Target, Sears and Children's Place. I found the interface a bit more clunky that Plastic Jungle and Cardpool, but the site had a wider range of retailers, including many that Plastic Jungle did not have, like AMC Theaters, H&M, Macy's, Payless and Zappos.

Costco

If you are a Costco member, you can get discounted gift cards to spas, restaurants, theme parks, gyms, movie theaters and more. The selection varies, and some cards sell out. Ask an employee for info on what cards are coming for the holidays.

Swagbucks.com

Join this free site and earn rewards as you go about your regular Internet search. Then trade in the rewards (called Swag Bucks) for gift cards. If you start now, you'll have enough to cover all your Christmas presents next year!

The Best Gift Card Bargains

So, which site has the best deals? The short answer is: It depends. However, I searched the sites for the best deals on gift cards from some the nation's top retailers. Here's what I found:

Note: These deals were good as of Nov. 18, 2012 at 9 a.m. Central.

Airlines: Get 7% off Delta Vacations gift cards at Plastic Jungle. You can also get 7% off American Airlines gift cards at GiftCardRescue. Cardpool didn't have any airfare gift cards in stock.

Best Buy: PlasticJungle had the best deal at 6% off. Cardpool's discount was 4.5% off. The card wasn't available at GiftCardRescue.com.

CVS: The biggest discount -- 6% off -- was at Plastic Jungle. Cardpool was a close second with 5.5% off. This wasn't available at GiftCardRescue.com.

Home Depot: With 7% off, Plastic Jungle was ahead of the 5.5% off offered at Cardpool.

Macy's: Get 9% off at GiftCardRescue as well as Cardpool. PlasticJungle did not have any Macy's gift cards in stock when I checked.

Movie theater chains: Get 15% off AMC Entertainment gift cards at GiftCardRescue. Get 12% off Regal Cinemas gift cards at Plastic Jungle. Get 9% off Fandango gift cards at Cardpool.

Payless ShoeSource: Get 18% off at Cardpool. You can also get 16% off at Plastic Jungle and 15% off at GiftCardRescue.

Sears: Cardpool took the lead on this gift card with 6% off. Plastic Jungle and GiftCardRescue did not have any in stock.

Target: Plastic Jungle has the best deal on Target gift cards, which is 5% off. Cardpool offered them at 4% off, while GiftCardRescue had them at a 3% discount.

Walmart: Cardpool edged out Plastic Jungle with a 2.5% off discount. PlasticJungle offered a 2% discount. It wasn't available at GiftCardRescue.

These offers can change often. For example, we'd previously seen deals for Zappos, J.Crew and Kmart gift cards at some of these discount websites. However, by the time this article was published, the deals had already disappeared.

Julia Scott founded the money-saving blog, BudgetBabe.com.