LOS ANGELES — Want to fool merchants with a fake ID? Hack someone's text messages? Or how about tracking where your co-workers are, without their knowing it?

There's an app for that.

The explosion in smartphone and tablet applications that allow people to check the weather, follow their stocks and play "Words With Friends" has a dark side — apps that facilitate questionable if not outright illegal behavior.

Apple's App Store, for example, offers "Drivers License" software that promises "unlimited access to realistic-looking licenses" for all 50 states. Though the phony licenses are advertised as entertainment, it's not hard to imagine a minor using one to try to get into a bar or a crook trying to pass a bad check.

Some members of Congress have pressured companies to remove certain apps, with mixed results. Banning apps isn't a viable option, according to legal experts, who say apps enjoy the same kinds of First Amendment protections as books, movies and music.

That means, in effect, that there are no restrictions on apps beyond the self-policing that companies like Apple and Google do to keep dubious products off their sites.

"The sky's the limit for developers, and that includes apps that may help people break the law," said Julie Samuels, an attorney with the Electronic Frontier Foundation in San Francisco, which advocates for consumer and privacy rights. "There's unfortunately the potential for criminality with new technology."

In response to questioning at a recent congressional hearing, Apple Vice President Bud Tribble declared that the Cupertino, Calif., company would never "allow apps that encourage illegal behavior."

But when contacted by the Los Angeles Times, Tribble and other Apple executives declined to discuss the Drivers License app, or the iBlunt app that also can be downloaded on Apple's site. It allows iPhone users to display an image of a marijuana cigarette — and when they blow into the device's sensor, the "joint" emits smoke.

Even if Apple or Google ban certain apps, they are readily available on underground or alternative sites. Among the products now available:

The Secret SMS Replicator app, available in several third-party online stores for Android apps, allows people to have text messages from someone else's phone forwarded to them automatically, and without their knowing it.

The company markets the app to parents who want to keep tabs on their young children. Adults can also use the software to ensure that a family member or friend receives a copy of their text messages — that's legal, as long as the person whose messages are shared gives permission.

But the software could also be surreptitiously installed on someone's phone — that of a boss, business rival or significant other — without that person's knowledge, as long as the installer can get access to the phone for a few minutes. ("Grab your boyfriend's phone while he is in the shower," app developer DLP Mobile recommends on its website.) Doing so without permission amounts to illegal wiretapping, according to experts.

The Stalqer (pronounced "stalker") app shows you where your friends or co-workers are at any given moment by culling location data from Facebook and plotting them on a map. For it to work, you must first be Facebook "friends" with your target. The developer is working on ways to harvest similar data from people's Twitter updates.

The Police Light app mimics a squealing police siren. News reports from around the country indicate that at least three people have used it or similar apps to trick drivers into pulling over. Like the others, this app by itself is not illegal — but someone using it on the highway could get in trouble for impersonating a police officer, which can be a felony.

Among the most controversial apps are the ones that warn drivers about drunk-driving checkpoints.

This year, U.S. Senate Majority Leader Harry Reid, D-Nev., and three other senators asked Apple Inc., Google Inc. and BlackBerry maker Research in Motion Ltd. to remove the DUI apps from their stores, saying such apps made it easier for drunk drivers to evade arrest, heightening the danger to other motorists.

Research in Motion and Apple complied, but Google refused. A Google spokesman said the apps did not violate its content policy but declined to elaborate.

Eric Fonoimoana of Hermosa Beach, Calif., doesn't see a problem. The 42-year-old real estate agent travels frequently around Southern California for work and relies on PhantomAlert, one of the DUI apps banned by the app stores, to ensure a smoother commute.

"With unemployment so high, it's incredible that the government is wasting time on smartphone apps," Fonoimoana said. "Sure, you can use it for bad purposes, but a lot of people just use it to avoid traffic. Checkpoints can really clog up the roads."

Jan Withers, president-elect of Mothers Against Drunk Driving, begs to differ. The apps are dangerous, she said, and need to be regulated.

"These are marketed for tipsy drivers to evade the police," she said. "Common sense tells us that most or many people who use it want to avoid getting arrested for drinking."

Courts have ruled that software code — which is what apps essentially are — is free speech and protected in much the same way as books, movies and music. The Supreme Court, in vigorously defending free speech, notably struck down a California law last month that banned sales of violent video games to minors.

"There has yet to be a big court case over apps, so you cannot say 100 percent that it would be protected as free speech," said Samuels of the Electronic Frontier Foundation. "But given how the court has ruled in the last few years, any law ... regulating apps would most likely be struck down."

John Morris, general counsel of the Center for Democracy and Technology, a digital rights advocacy group in Washington, said regulating apps is akin to "playing Whac-A-Mole."

"As soon as you regulate one type of app, another will spring up that you have never even thought of," he said. "Lawmakers would always be playing catch-up."

Apple, Google and Research in Motion do set certain guidelines for developers who want to sell apps through their official digital storefronts. Most ban apps that violate copyrights or trademarks, and some distinguish on matters of taste.

The Apple App Store and the Android Market, for example, ban pornography. BlackBerry's guidelines for developers says it will reject apps for "inappropriate" content, including anything that is "abusive, belittling, harassing, deceptive or malicious," but company executives declined to elaborate.

Android and BlackBerry also allow developers to operate unofficial digital storefronts where banned or rejected apps can be sold. Even Apple, which does not allow apps to be sold outside of its official App Store, has competition from black-market app stores.

Cydia, the largest of the underground Apple stores, has thousands of rejected or banned iPhone and iPad apps for sale, including iSnort, for simulating snorting cocaine, and one for digitally defacing religious icons called Me So Holy.

Founder Jay Freeman said Apple was like "an anticompetitive Big Brother," with apps often banned for mysterious and unexplained reasons.

But Freeman does not allow certain kinds of apps, such as child pornography, into the store.

"Even I have to draw the line somewhere," he said.