Learn how to avoid the hidden costs of "free" apps.

By Susan Stellin

The rip-off: Just because a game is free doesn’t mean it won’t cost you. Some companies make so-called velvet-rope apps for the iPad, the iPhone, and the iPod Touch. With these, a player has an easier time advancing if she spends some money, says Jinny Gudmundsen, the executive editor of apps and video games at Common Sense Media, a media review and advice site.

Certain items, like a $25 barrel of Smurfberries in the popular Smurfs’ Village or $50 worth of toxin in Zombie Cafe, can be bought by a user who enters a password. (A credit-card number isn’t required, since the device connects to the iTunes store.) Recently Google introduced an in-app–purchase option for its Android operating system, so users of those devices may be susceptible to similar charges in the future.

The tip-off: Free apps are more likely than paid apps to charge for items that you can buy within the game, says Molly Wood, an executive editor of CNET.com, a technology site. The games Tap Zoo and Dolphin Play have drawn criticism for their in-app charges, which can run as high as $30 a purchase. Low-cost games tempt users, too: Players of Bejeweled 2 can spend $9 on 1 million coins, which are redeemable for power-ups in the game.

How to protect yourself: In response to irate parents who were hit with surprise charges on some of the aforementioned games, Apple recently changed its operating system to require a user to enter the password a second time before making a purchase. However, that doesn’t fully fix the problem. “Many people are still vulnerable if they haven’t updated their operating software to 4.3 or later, or if they have given their children their password, as many parents do,” says Wood.

Another option: Disable in-app purchases on your Apple device by choosing Settings>General> Restrictions>Enable Restrictions and creating a unique password, then scrolling to Allowed Content and turning off in-app purchases. Or put the phone in airplane mode before letting your child play with it. “This blocks all downloads,” says Wood. In other words, game over.