Let's do a review.
Stepmania is a still-thriving Dance Dance Revolution PC clone, with the ability for users to take their own MP3s and make custom levels based around them. Osu! does the same with the Ouendan/Elite Beat Agents playstyle, and Frets on Fire with Guitar Hero.
On the professional side, Phase, from Harmonix (the same people who brought us Guitar Hero and Rock Band) automatically analyzes your songs to make a game that is essentially a simplified version of their premier series for your iPod. Beats does the same for the PSP. As does Audiosurf for the PC.
Combine this with dozens of amateur Flash games, and it becomes obvious that music games supplied by the input of the masses is getting big. And the next big step may come with Rock Band, as a Harmonix rep has mentioned that they are interested in a similar system being released for the game at some point.
In other words, get those MP3s, check these programs out, and have fun making your own levels.