Osu!

[NightRaid104]

Fun rhythmic game, based mainly on music.
features completly player-made Beatmaps created using music and timing
has community of 1.12 billion ranked users
there are 4 known modes for it
Solo : point and click in time
Taiko : 4 keys hitting like a drum in time
Catch the beat (ctb) : using arrow keys to catch fruits that go with the music
mania : using 4-7+ keys, kinda like playing a simplified piano using a key board

I've been playing this game for the past few years on this shitty computer with only 1.6ghz and it still works well,
this game is Free and I believe it always will be, but you can donate. I've been playing only in ctb since its the only one I can really play but either than that I love this game and if it were to shut down completely I wouldn't know what to do from then on.

http://osu.ppy.sh/

songs mainly feature Asian(or international) music but still has lots of western music as well along with game music (even 8-bit game songs). Osu is played world wide and has had many live streaming competitions