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.

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.

Tempest jQuery Templating Plugin

The JavaScript InfoVis Toolkit provides tools for creating Interactive Data Visualizations (graphs, trees, charts...) for the Web.

Piston is a Django mini-framework creating APIs.

Interesting point. I guess I'm only using about 5% of Python's standard library on a daily basis.