osu! is a free rhythm game for the PC that is somewhat in the style of DDR except with clicking.
I discovered this game after watching a Starcraft II stream where the player was warming up with osu! before playing his matches.
There is a built-in editor that allows you to create your own beatmaps, which has resulted in a large selection of user-generated content. Plus, there are online rankings and even multiplayer mode!
I’m pretty impressed so far with what I’ve seen, and it looks like this game will last me quite a while considering that normal difficulty is challenging enough already.