Personally, I enjoy using VIM (MacVim on OS X to be specific), but I'd also suggest TextMate, as well as TextWrangler, for those who don't want to pony up the $49 for TextMate.
On the Windows front, I'd suggest Notepad++ for beginners. Although it doesn't provide a language reference by default, I think it functions as a great replacement for the Notepad application that comes packaged with Windows.
JSFiddle also looks to be a very powerful tool (I hadn't heard of it until today).