Guitar Hero and now Rock Band have people asking for an encore. The downloadable (for a fee) songs for these two games have done so well that they have made an impact on online music sales as a whole. During the first two months of sales of Rock Band, MTV and Harmonix have sold 2.5 million songs, meanwhile Activision has sold 5 million songs for Guitar Hero since November, which is quite impressive considering that the user base for these games is quite limited.
Anyone who has played Guitar Hero or Rock Band (and you should) can tell you how fun and challenging these games can be. Since you really play to improve your skills as a rock star, once you have perfected a song it is only natural you move on to the next one and so on until you finally conquer that last song, but due to the the downloadable content now available there doesn't have to be a 'last' song. Meanwhile Harmonix is certainly cashing in on it.