html5 compatibility chart
Traceur is a JavaScript.next-to-JavaScript-of-today compiler that allows you to use features from the future today
a reusable non-linear personal web notebook
The Computer Language Benchmarks Game is a collection of 429 programs, consisting of 13 benchmark reimplemented across 33 programming languages. It is a fantastic resource if you are trying to compare programming languages quantitatively. Which, oddly, very few people seems to be interested in doing.