CSS Code Generation
CSS generation tool. Type in all the values you want and it makes the CSS for you automatically.
Generic sandbox for js, html, css.
See how good your JS is.
http://placekitten.com/x/y displays a placeholder kitten width x height y. Y is optional, it'll draw a square kitten without it, dimension x.
Tells you where the browser is spending its time. Vital for performance tuning.
Very comprehensibe HTML5 Scalable Vector Graphics demo.