Osu! Is a pretty fun time waster. A lot of you have probably heard of it as it's been around for quite a few years. It still has a pretty big community, too.
Made a couple videos to show you the gameplay, it involves beatmaps for songs. (Don't worry, the below videos are NOT monetized, so even if you don't wanna give me money, you can still watch em.)
Osu! Heartbeatz:
Osu! Heartbeatz - easyOsu! Melt:
Osu! Melt - NormalIt's a LOT harder than it looks.