Triptych calls JsFiddle an amazing tool that is accessible anywhere via a browser. It allows you to rapidly create demos, examples, bugs, and many other web-based applications and sites.
This is the most valuable tool I have found for hunting down and killing that one little glitch.
IF you would rather that all of your development files not be posted to the internet this is the editor for you. It is fast, has syntax highlighting for tons of languages, autocomplete, tabs, and much more. My favorite tool.