Apache Commons - A Staple in the Java Open Source World

Apache Commons is a staple in the Java open source world.

commons-beanutils.jar (contains everything to do with beanies)
commons-collections-3.2.1 (contains apache commons collections)
commons-lang-2.4.jar (contains additional classes for java.lang)
commons-logging.jar
commons-CLI.jar (command-line parsing)

There is also a Commons sandbox of projects-in-the-making. Amongst these are:

CLI2 - redesign of Commons CLI
CSV - module for reading/writing CSV
OpenPGP - for signing and verifying data using OpenPGP
Javaflow - to capture the state of an application. Means of "checpointing" an application using the idea of "continuations" borrowed from Scheme.

Commons dormant contains stuff no longer maintained, like Clazz for introspection, and Cache for object caching services.