Do you have a favorite site or homegrown page in your toolbox to help you during development of your Javascript?
Something to help you:
Looking for somewhere to paste my JS into, click a Run button, and have it evaluate the statements. This might be for simple snippets for manipulation of numbers, strings, custom objects, etc.
JSLint helps a lot for validating JS.
In terms of running/debugging, I use Firebug on Firefox, and Opera/Safari/IE8's built in JS debuggers. (None of these are online, but all are pretty powerful.) I think that Firebug Lite would make for a decent online JS debugger (in that it's a bookmarklet instead of a plugin or specific browser feature).
It's a Firefox extension, not an online service, but you should look into Firebug if you need to execute arbitrary JavaScript for debugging. Or Firebug Lite if you need support for other browsers.
If you're using a WebKit based browser, (Safari or Chrome), then you can use the Web Inspector. I think that IE 8 and Opera also have equivalent features.
If you need to run JavaScript from the command line, you can use SpiderMonkey directly; it has a JavaScript REPL that you can run.
Give a look to JsBin it's a great online tool for collaborative JavaScript debugging...
You can easily incorporate the most popular JavaScript libraries, and you can even use it for Ajax Request Debugging...
Check the introductory videos...