My current favourite online tool is jsfiddle. This is an incredibly well designed tool.
If you ever find yourself wanting to quickly try out some HTML, CSS or javascript, without any edit-save-publish-reload cycle, then jsfiddle is the tool for you. It keeps surprising me how well put together it is. Sharing snippets of work with friends and colleagues just requires cutting and pasting a simple URL. Forking is literally a one-click process. Versioning is built in so seamlessly you hardly notice it. And although you can set up an account to manage your code snippets, you don’t need to login to start using it.
This is a lesson to me on user-centric design. The barrier to entry is absurdly low, and there are a bunch of powerful features that are easily discoverable when you want to go beyond the basics. You have the option of pre-loading any number of popular javascript libraries such as jQuery and Prototype, and even Coffeescript is supported.
So, go give it a try. jsfiddle could quite easily be integrated into your development processes in the next ten minutes.