YouTube Inc. : Or really, any video at all. According to Cisco Systems Inc. (Nasdaq: CSCO), video will overtake peer-to-peer traffic for the first time by the end of this year. By 2014, video will hit 767 exabytes, or more than three-quarters of a zettabyte, of data usage. So, it doesn’t matter what app you are using, if it has video involved, you’re at risk -- which certainly makes the prospect of a Hulu LLC mobile app a little less enticing. (See Cisco: Video Traffic Set to Dominate Internet.)

It’s really video, more so than any other possible app that’s driving data usage, says Vantrix's Lopez. YouTube’s mobile Website alone accounts for more than 50 percent of video traffic, but mobile users are also starting to watch full-length movies and mobile video for extensive periods of time. Viewing sessions will only continue to increase in time, he warns.

Knocking Live: This peer-to-peer live streaming video app has already surpassed 1 million users. The app lets users share cool videos with their friends of, say, an iPhone in flight, but it also causes data traffic to skyrocket.

ABC Inc. TV: The app, which supports 3G streaming, has proven popular on the iPad. But, if it only takes a few YouTube videos to max out, ABC's focus on streaming high-quality videos and longer-form content could put you over the top in one day alone. (See ABC Finds Success on iPad.)

Pandora Media Inc. : AT&T’s handy Data Calculator shows music streaming to be a significant source of data consumption, so any streaming music app will cost you.

Wired: On the iPad, Wired's magazine app is pretty slick, but it also requires a 500MB download, more than 100 times larger than the average song downloaded from iTunes. Simply downloading this app will be more than enough to exceed the 250MB minimum monthly data plan for the iPad. (See International Sales Push iPad Over 2M Mark.)

— Sarah Reedy, Senior Reporter, Light Reading Mobile