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