With so many languages and frameworks that exist, and with new ones appearing all the time, I don't have the time to download, install, and configure each one to evaluate it. In the past I've run across webapps that allow one to write or paste code into a window, and see the results in realtime in the browser, usually in a tutorial setting.

What are your favorite sandbox sites for a given technology?

Edit: @fretj provided the link to the excellent Google Code Playground (+1 upvote), but I thought that it was just for experimenting with Google's own apps (Search, Maps, Earth, Language, etc). But it turns out that it contains a few hidden gems: In addition to their apps, you can try out the many Javascript libraries that they host including jQuery, jQuery UI, MooTools, Dojo, and Prototype Scriptaculous.

They're all hidden under the Libraries category in the "Pick an API" box. I overlooked the category because I thought it was for an app called Google Libraries. There's also a Javascript category for Javascript itself.