What do you use to minimize and compress JavaScript libraries?

I use YUI Compressor. Seems to get the job done well!

I've used YUI Compressor for a long time and have had no problems with it, but have recently started using Google Closure Compiler and had some success with it. My impressions of it so far:

• It generally outperforms YUI Compressor in terms of file size reduction. By a small amount on simple mode, and by a lot on advanced mode.
• Simple mode has so far been as reliable as YUI Compressor. Nothing I've fed it has shown any problems.
• Advanced "compilation" mode is great for some scripts, but the dramatic size reduction of your script comes at the expense of a lot of meddling with your code that stands a decent chance of breaking it. There are ways to deal with some of these problems and understanding what it's doing can go a long way to avoiding problems but I generally avoid using this mode.

I've moved over to using Google Closure Compiler in simple "compilation" mode, because it slightly outperforms YUI Compressor in general. I have used it considerably less than I have YUI Compressor but from what I've seen so far I'd recommend it.

One other that I've yet to try but sounds promising is Mihai Bazon's UglifyJS.