Tapulous, developer of the popular Tap Tap Revenge series of iPhone-based music games, is adding a broad range of independent music as downloadable content and other new features to the most recent installment of the franchise—Tap Tap Revenge 3.

For starters, the company is making the game free. When it was first released in October, the game cost $1 because at the time, Apple would only allow paid apps to sell additional content. Apple since eliminated that requirement, so now Tapulous can both give away the main game and sell extra downloadable songs for fans to play along to.

Along with that effort comes an expansion of content into the indie channel, with new content coming from Big Sounds International, Black Hole Recordings, Bug Music, Downtown Records, Fearless Music, Nettwerk Records, Quango Records, Rocket Science and Ultra Records.

In addition to making music from these labels available as unlockable extras in solo game mode, Tapulous will create custom game rooms for each company where multiple fans playing the game can play the same songs together in multiplayer mode, chat about the songs, and access promotional links to both the artists and the labels involved such as Facebook and direct Web sites.

According to Tapulous head of business development Tim O’Brien, the moves are meant to increase fan engagement with the Tap Tap Revenge 3 game in hopes of further broadening its appeal as a music platform. O’Brien says the franchise has been installed more than 20 million times, and is played more than 2.5 million times a day. Fans have bought and downloaded more than 1 million extra songs to add to the game since it came out in October.