Dive Into HTML5 has an in-depth explanation of the Geolocation API at http://diveintohtml5.info/geolocation.html, whereas the MozDev article “Orientation and Motion Data Explained” gives a good overview of three-dimensional orientation and movement: https://developer.mozilla.org/en-US/docs/DOM/Orientation_and_motion_data_explained/.
The Fullscreen API is explained in the Sitepoint article “How to Use the HTML5 Full-Screen API” by Craig Buckler, although the API changed slightly as I was writing this, so some object names or properties may have been updated. You can find the article at http://www.sitepoint.com/html5-full-screen-api/.
The Battery Status API is well explained by David Walsh at http://davidwalsh.name/battery-api/, and a discussion of the previous and newly updated Network Information API is at http://nostrongbeliefs.com/a-quick-look-network-information-api/.
HTML5 Rocks gives the best explanation of getUserMedia() in their article “Capturing Audio & Video in HTML5”: http://www.html5rocks.com/en/tutorials/getusermedia/intro/. The full aims of the WebRTC project are listed at http://www.webrtc.org/.
MozDev (again) gives a concise introduction to the Web Storage API: https://developer.mozilla.org/en-US/docs/DOM/Storage/.
The most accessible guide to the Drag and Drop API that I found was written by the HTML5 Doctors at http://html5doctor.com/native-drag-and-drop/, while the five-part “Working with Files in JavaScript” by Nicholas Zakas is an excellent resource for the File API: http://www.nczonline.net/blog/2012/05/08/working-with-files-in-javascript-part-1/.
The APIs that form the Firefox OS project are listed at https://wiki.mozilla.org/WebAPI/, and the slides from the presentation “WebAPIs and Apps” by Robert Nyman provide a great overview of the APIs: http://www.slideshare.net/robnyman/web-apis-apps-mozilla-london/. “Are We Mobile Yet?” gives an at-a-glance guide to levels of API implementation: http://arewemobileyet.com/.