Katy Perry's Revenge 2: App tops iTunes chart

A free, two-song version of Katy Perry's Revenge has rocked its way up to the top of the App Store charts. Sponsored by Microsoft's search engine Bing, Katy Perry's Revenge 2 lets you tap your way through Perry's smash song "Firework" and her latest hit, "E.T.," featuring Kanye West.In case you've never played a Tap Tap Revenge game before, you're challenged to tap one of three colored orbs at the correct time, as it passes through a circle near the bottom of the screen. Similar in concept to Guitar Hero and Rock Band, you'll rack up points for staying in rhythm as long as you can without a mistake, pressing and holding some notes for a longer period of time and collecting multipliers for bonus points.

Behind the action are images of Katy Perry and graphics related to the song (such a seeing animated fireworks at key times throughout the song).

This free app – which might make you watch a Bing advertisement before you play (it seems to be random) – also lets you access Katy Perry lyrics from Bing, read tour info and follow her tweets – all from within the app.

As with many other free apps, there's also an option to upgrade to the full Katy Perry's Revenge game ($4.99) with many more of her songs -- including chart-topping tracks like "I Kissed a Girl," "California Gurls," "Hot N Cold," "Teenage Dream," and "Waking up in Vegas."