Three days after its release last July, Tap Tap Revenge -- an iPhone game that's basically Dance Dance Revolution except that you tap your fingers to a song rather than dance to it -- shot to No. 1 among free game downloads on iTunes. It was testimony to the appeal of playing along to bands such as Coldplay and Dave Matthews Band. Within a week, artists were reaching out to Tapulous, the game's developer. Because fans can click through to purchase music from iTunes, CEO Bart Decrem notes, the retail ripple is quantifiable -- for instance, 50,000 players headed to iTunes to check out Katy Perry's "Hot N Cold" in October. Decrem has partnered with labels (EMI) as well as individual acts (Christmas With Weezer, anyone?) to create band-branded apps. -- by Kate Rockwood