Debug interfaces...

Does anyone have any particular debug interfaces (or classes) that they find easy and useful to use in your own stuff? It's come to a time in my project where some nice organized debug code is pretty needed, and sporadically placed System.err.println()'s aren't cutting it..

(btw - I'll be sure to mention when I've found out how well the java.util.Preferences workaround y'all mentioned works, but it's embedded pretty deep in my buggy code at this point, so I don't really know yet [yeah, I know, I should have a test harness for each of my classes...once I get debug code in there, I'll do that too... :P] )