If you liked Elite Beat Agents, then you'll love OSU!
"osu! is a rhythm game based on the gameplay of the popular Nintendo DS series Osu! Tatakae! Ouendan! and its overseas counterpart Elite Beat Agents. While trying to keep an authentic feel gameplay-wise, osu! has a wide range of online features, such as player rankings and replay exchange. Also available is a fully functional in-built editor allowing intuitive creation of your own beatmaps, and a quickly growing library of user-created content."
Not only is it free, but it is challenging! It may be tough at first, but I think once you get the hang of it, you'll love it. I play with my tablet, but some of the best players use their mouse! It's amazing!
The great thing about osu! is that user submitted maps are always updated, allowing infinite replay value. Also, there is online ranking, different difficulties for maps, and a great community. Some of the team may even be making a port to the DS and iTouch (Don't ask them about it though, enough people do as it is).