Drawing and manipulating graphics
Audio and Video APIs like HTMLMediaElement, the Web Audio API, and WebRTC allow you to do really interesting things with multimedia such as creating custom UI controls for playing audio and video, displaying text tracks like captions and subtitles along with your videos, grabbing video from your web camera to be manipulated via a canvas (see above) or displayed on someone else's computer in a web conference, or adding effects to audio tracks (such as gain, distortion, panning, etc).
Device APIs are basically APIs for manipulating and retrieving data from modern device hardware in a way that is useful for web apps
GeoLocation APIs to get the location of the User
Notification API to send notifications of events to the User
A complete list of such APIs can be found on https://developer.mozilla.org/en-US/docs/Web/API