UglifyJS online demo
The UglifyJS2 API documentation pages are more or less complete. I also worked on an online demo which allows you to minify scripts from your browser; the intended use case is just to demonstrate features of UglifyJS like compressor/beautifier options and source maps. You can minify a script and click some token in the minified output and the editor will move to the original position of that token in the initial code. You can easily see all the warnings that UglifyJS dumps, and click them to go to the original location and fix them.
It's only tested with Chrome and Firefox. Should work with Safari too, but I'm pretty sure it won't work in Opera or IE. Try it out! If it doesn't appear to work and you visited this website recently, please clear the cache. If you have any comments or suggestions I'd love to know about it!