Possibly UglifyJS? It's the minifier that the jQuery project is currently using.
Check out these speed comparisons.
See the statistics src: http://axod.blogspot.ca/2010/01/google-closure-compiler-advanced-mode.html
compare JSMin, by Douglas Crockford and Yahoo! YUI
Minifier | Bytes |% of original| gz Bytes | gz % original None | 428,264 | 100% | 91,750 | 100% JSMin | 249,372 | 58% | 57,338 | 62% YUI | 235,214 | 55% | 55,990 | 61% Closure (STANDARD) | 219,446 | 51% | 53,515 | 58% Closure (ADVANCED) | 166,774 | 39% | 47,372 | 52%
Just for reference, you can access the Google Closure Compiler UI here http://closure-compiler.appspot.com/home
Having noted that, I just ran Closure Compiler in standard mode against a library I wrote, https://github.com/robertdmunn/gadget-ui, to compare the output with UglifyJS, which I am using through Grunt. The base code is 20KB, Uglifier and Closure Compiler (std mode) both made it 10KB. For most use cases, I see that as good enough. Closure advanced mode only reduced it to 9KB. For edge cases - web scale - 10% smaller is decent. Consider, though, that gzip compression will reduce file size by 60-80% in my experience, so the 10% reduction using advanced mode only saves you 2-4% above standard mode if you use gzip. 2-4% is valuable at web scale, but for most developers, it might not be worth the potential problems introduced.