The Apache Commons Lang Library.
Distributed on the Apache License, Version 2.0
Apache Commons Lang provides a host of helper utilities for the java.lang API, notably String manipulation methods, basic numerical methods, object reflection, concurrency, creation and serialization and System properties. Additionally it contains basic enhancements to java.util.Date and a series of utilities dedicated to help with building methods, such as hashCode, toString and equals.
Note that Commons Lang 3.0 (and subsequent versions) use a different package (org.apache.commons.lang3) than the previous versions (org.apache.commons.lang), allowing Commons Lang 3 to be used at the same time as Commons Lang 2.