Apache's Log4j is pretty popular among open source community. I've been using it for about six months already. They got pretty good documentation and you can find other resources related to Log4j easily as well.
Co-author of SCMAD Exam Guide, Author of JMADPlus SCJP1.2, CCNA, SCWCD1.4, SCBCD1.3, SCMAD1.0, SCJA1.0, SCJP6.0
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