A very cool widget that shows the history of the number of times your site has been tagged on delicious, and also displays the top tags used.

Quoted: Keep, share, and discover the best of the Web using Delicious, the world's leading social bookmarking service.

Closure compiler - list of errors and warnings.

Many of the JSDoc tags are also used as hints to the Closure javascript optimizing compiler.

You can use Google's new JavaScript minifier, Closure, here.

Besides whitepace compression, it shortens the names of local variables, and, in the Advanced mode, can remove unused code.

I think it has some built-in knowledge of the Closure Library too, so if you use that you can pull in only the parts that are used?

UPDATE - after playing with this for a few minutes, it's clear that Closure is WAY MORE than a minifier. Even in the Simple mode it can do unused code elimination, code folding, common subexpression elimination, unwinding function calls, etc.!

Closure's unit testing framework looks pretty sweet.

Interesting that Closure is not passing all it's tests on Firefox right now (I get 17 failures of 312 tests).

Google has been using this internally for some years to help with their JavaScript development. Now they have released it as an open source project.

A JavaScript code optimizer from Google! Didn't Microsoft Research recently announce a JavaScript on-demand loading optimizer too?

Quoted: The official Google developer blog. The latest news on Google APIs and developer tools.

Quoted: Official home of the JS/CC project, an implementation of a LALR(1) parser generator for JavaScript in JavaScript itself.