Javascript is great fun to play with in one's browser - you can edit, and reload and voila :)

Recently i've been playing with the HTML5 canvas, drawing sequence diagrams and network diagrams programmatically - and can't remember when i last had so much fun programming - took me back to my teenage years and my first programs, drawing "graphics" on those cathode ray tubes ;)

Being a biased Java developer, i would otherwise recommend Groovy.