Live coding: http://www.mrdoob.com/projects/htmleditor/ WebRTC: http://cbateman.com/demos/head-coupled/ (needs well-lit room to track your eye position) 3D: http://patapom.com/topics/WebGL/cathedral/index.html
And of course there's plenty more that can be done. People tout an IDE like Lighttable, which of course uses web technology to accomplish everything special it does.
Edit - and as ugly as Javascript is, it's pretty powerful. Has all the power of Lisp/Scheme, beat with a C-flavoured ugly stick for awhile, enhanced with advanced runtimes (V8), and transformed by transpilers/compilers, is even used to represent bytecode (asm.js) and emulate other platforms (http://fir.sh/projects/jsnes/ and http://copy.sh/v24/)
Edit2 - and of course, Smalltalk in the browser: http://amber-lang.net/ and a Lisp machine-type thing in the browser: http://alex.nisnevich.com/ecmachine/
Edit3 - WTF how did I not see this... http://idflood.github.io/ThreeNodes.js/public/index.html