Popular libraries used by developers:
- log4j - Logger, needy tool for debugging.
Tip: Put the configuration file "log4j.properties" in /Tomcat 5.5/common/classes/ to be shared among the webapps
- WURFL - Identifying devices capabilities for mobile development (eg. WAP)
- BouncyCastle - For encryption
- ProGuard - Obfuscation not only protects your code, but also make the package smaller.