# Web Audio
Spectrograms and synths, this one is fairly mature, no IE support of course, but everything else supports it.
# MIDI
Mostly fun with knob and slider controls for various sideprojects. No support anywhere save Chrome.
# WebVR
Works surprisingly well, I find es6 to be much more friendly when validating random VR ideas than Unity or Unreal, which are basically the only other options. Requires a custom build of Chrome or dev Firefox, also works in Cardboard via polyfill.
# Gamepad API
Self explanatory, some games are just more fun with gamepads, now those games can be webgames. Works pretty much everywhere
# WebGL2
Transform feedback, more than one Render Target (deferred rendering with just one requires some extremely clever swizzling that's really not that fun to work with/implement), Instancing, Uniform Buffers and more. Tons of fun stuff that will bring the state of 3D graphics in the browser a lot closer to today. Behind a flag in Chrome, supported in Firefox.