There's a bevy of sandbox and testing sites for nearly every language imaginable, I've taken to compiling a list at http://www.jkirchartz.com/2012/06/rocking-stackoverflow-sites-to.html
any software for working offline just like codepen and jsfiddle
You forgot this awesome site -:
@vijayant I just added it, thanks!
@jkirchartz most people may already know about play.golang.org but its a good resource. It can also be ran locally using godoc.