CSSDesk is an Online CSS Sandbox. It allows people to quickly test snippets of CSS code, and watch the result appear live. I had very high ambitions for the project. There are line numbers and syntax highlighting appear live in the text box as you type. One of the greatest things is that it allows users to share their code with others as well.
And also, if you haven’t heard about Google’s Code Playground, it’s a web-based tool that lets web developers try out all of the APIs that Google provides, tweak the code, and see the results. Google’s Code Playground lets you play around without opening an external editor, and all of the APIs are loaded for you in the Pick an API box.