Underscore is a utility-belt library for JavaScript that provides a lot of the functional programming support that you would expect in Prototype.js (or Ruby), but without extending any of the built-in JavaScript objects. It's the tie to go along with jQuery's tux.

I disagree.

Bookmarked for future use. Haven't tested it yet.

Skulpt is an entirely in-browser implementation of Python.

No preprocessing, plugins, or server-side support required, just write Python and reload.

“There is no 'Webkit on mobile!'”

Raphaël is a small JavaScript library that should simplify your work with vector graphics on the web. If you want to create your own specific chart or image crop and rotate widget, for example, you can achieve it simply and easily with this library.

"In Secrets of the JavaScript Ninja, JavaScript expert John Resig reveals the inside know-how of the elite JavaScript programmers."

Nolan (in the comments) on the topic of language mix in web apps: “I have a feeling, though, that the JS only frameworks will be big players very soon, as it easily serves as a lingua franca on the web.”

If you are a web developer you need to know