I just was in the need of an java-based argument parser.
That's what I found:
* JSAP (pretty active; seems to be the most advanced incl. usage generation, support for datatypes and Unix-style cascaded configuration files with default values)
* jcmdline (not as active as JSAP; aims for standardized commandline options, allows more sophisticated validations, displays usage and error messages)
* JArgs (not much active, very simplistic and easy to use)
It's hard to say which one is the best, since all suite different needs.
If you wonder, for my task I chose JSAP, but I'll keep an eye on all of them.
Phil Wilson pointed me to one more:
* Apache Jakarta Commons CLI (seems to be similar in functionality and usage like JSAP)