For those who love their music, you might want to take a look at Clone Hero (they're working on a better name) which aims to be a cross-platform clone of Guitar Hero.
They're releasing it for free, with the latest update only being released yesterday.
I've tested it with the Steam Controller and surprisingly, it actually works pretty well. My complete lack of skill is quite apparent when trying it because I seem to have no coordination at all. Telling my fingers to move that quickly is like turning on an energy saving bulb and and finding what you need before it's actually lit, but I can certainly appreciate how cool it is to have games like this that support Linux. It's quite impressive indeed!
They're even planning to add in online multiplayer, although they're aware that's a big undertaking and may take a long time.
If you're specifically after something open source, there's also Frets on Fire, but it seems that hasn't seen an update for years. There is also Frets on Fire X, a fork which aims to update it and add features that sees a little activity.
Thanks for the tip aFoxNamedMorris.