Closure Compiler

The Closure Compiler is a tool for making JavaScript download and run faster. It is a true compiler for JavaScript. Instead of compiling from a source language to machine code, it compiles from JavaScript to better JavaScript. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls.

http://code.google.com/closure/compiler/
http://code.google.com/closure/compiler/docs/error-ref.html

7 Free Tools to Minify your Scripts and CSS

1. JSMin (JavaScript Minifier) - removes comments and unnecessary whitespace from JavaScript files
2. JSO (JavaScript Optimizer) - allows you to manage your JavaScript and CSS resources and to reduce the amount of data transfered between the server and the client.
3. Packer – An online JavaScript Compressor
4. JSCompress.com – Online tool that uses either JSMin or Packer to compress your files
5. CSS Compressor – Online tool that compresses your CSS file
6. DigitalOverload JavaScript Minifier – Online tool that minifies your JavaScript files
7. YUI Compressor – A JavaScript minifier designed to be 100% safe and yields a higher compression ratio than most other tools.

http://www.devcurry.com/2009/11/7-free-tools-to-minify-your-scripts-and.html