Best JavaScript compressor

I recently released UglifyJS, a JavaScript compressor which is written in JavaScript (runs on the NodeJS Node.js platform, but it can be easily modified to run on any JavaScript engine, since it doesn't need any Node.js internals). It's a lot faster than both YUI Compressor and Google Closure, it compresses better than YUI on all scripts I tested it on, and it's safer than Closure (knows to deal with "eval" or "with").

Other than whitespace removal, UglifyJS also does the following:

• changes local variable names (usually to single characters)
• joins consecutive var declarations
• avoids inserting any unneeded brackets, parens and semicolons
• optimizes IFs (removes "else" when it detects that it's not needed, transforms IFs into the &&, || or ?/: operators when possible, etc.).
• transforms foo["bar"] into foo.bar where possible
• removes quotes from keys in object literals, where possible
• resolves simple expressions when this leads to smaller code (1+3*4 ==> 13)

PS: Oh, it can "beautify" as well. ;-)

Revisiting this question a few years later, UglifyJS, seems to be the best option as of now.

As stated below, it runs on the NodeJS platform, but can be easily modified to run on any JavaScript engine.

--- Old answer below---

Google released Closure Compiler which seems to be generating the smallest files so far as seen here and here

Previous to that the various options were as follow

Basically Packer does a better job at initial compression , but if you are going to gzip the files before sending on the wire (which you should be doing) YUI Compressor gets the smallest final size.

The tests were done on jQuery code btw.

• Original jQuery library 62,885 bytes , 19,758 bytes after gzip
• jQuery minified with JSMin 36,391 bytes , 11,541 bytes after gzip
• jQuery minified with Packer 21,557 bytes , 11,119 bytes after gzip
• jQuery minified with the YUI Compressor 31,822 bytes , 10,818 bytes after gzip

@daniel james mentions in the comment compressorrater which shows Packer leading the chart in best compression, so I guess ymmv