Piotr Zalewa has created a really great playground, jsFiddle, for testing sample code and playing with the Web. With an area for the holy trinity of the Web (HTML, CSS, JS) and an output region, you can get right to hacking.
You can also check out the examples and see great stuff such as Processing in action.
And the finishing touch, share and embed.
Piotr wrote all of this using CodeMirror and MooTools. Nice! Having worked on Bespin, and developed a playground like this (looking forward to show a new mobile one soon!) I appreciate the work!