The Digester package lets you configure an XML to Java object mapping module which triggers certain actions called rules whenever a particular pattern of nested XML elements is recognized.

Apache Commons Validator provides the building blocks for both client-side and server-side data validation. It may be used standalone or with a framework like Struts.

The Commons Configuration software library provides a generic configuration interface that enables an application to read configuration data from various sources and requires Java 8.

Apache Commons DBCP software implements Database Connection Pooling

Usefull class adapters based on ASM, a very small and fast Java bytecode manipulation framework

Spring Data makes it easy to use data access technologies, relational and non-relational databases, map-reduce frameworks, and cloud-based data services.

ojAlgo and Apache Commons Math integration

The Apache Commons CSV library provides a simple interface for reading and writing CSV files of various types.

Commons-IO contains utility classes, stream implementations, file filters, and endian classes.

Apache Commons JEXL is a library that enables scripting features in Java applications and frameworks..