Yuicompressor-old is a project mainly written in Java, it's free.
A refactored version of the yuicompressor from a while back. Might implement my changes on the newest version. See the fork.
== yuicompressor revision
Allows compression of multiple js and css files with a single execute.
java -jar yuicompressor-2.4.2-netbooks.jar
The jar can be found under the build directory.