Apache Commons

Components  

Attributes	Runtime API to metadata attributes such as doclet tags.
BeanUtils	Easy-to-use wrappers around the Java reflection and introspection APIs.
Betwixt	Services for mapping JavaBeans to XML documents, and vice versa.
Chain	Chain of Responsibility pattern implemention.
CLI	Command Line arguments parser.
Codec	General encoding/decoding algorithms (for example phonetic, base64, URL).
Collections	Extends or augments the Java Collections Framework.
Compress	Defines an API for working with tar, zip and bzip2 files.
Configuration	Reading of configuration/preferences files in various formats.
Daemon	Alternative invocation mechanism for unix-daemon-like java code.
DBCP	Database connection pooling services.
DbUtils	JDBC helper library.
Digester	XML-to-Java-object mapping utility.
Discovery	Tools for locating resources by mapping service/reference names to resource names.
EL	Interpreter for the Expression Language defined by the JSP 2.0 specification.
Email	Library for sending e-mail from Java.
Exec	API for dealing with external process execution and environment management in Java.
FileUpload	File upload capability for your servlets and web applications.
IO	Collection of I/O utilities.
JCI	Java Compiler Interface
Jelly	XML based scripting and processing engine.
Jexl	Expression language which extends the Expression Language of the JSTL.
JXPath	Utilities for manipulating Java Beans using the XPath syntax.
Lang	Provides extra functionality for classes in java.lang.
Launcher	Cross platform Java application launcher.
Logging	Wrapper around a variety of logging API implementations.
Math	Lightweight, self-contained mathematics and statistics components.
Modeler	Mechanisms to create Model MBeans compatible with JMX specification.
Net	Collection of network utilities and protocol implementations.
Pool	Generic object pooling component.
Primitives	Smaller, faster and easier to work with types supporting Java primitive types.
Proxy	Library for creating dynamic proxies.
Sanselan	A pure-Java image library.
SCXML	An implementation of the State Chart XML specification aimed at creating and maintaining a Java SCXML engine. It is capable of executing a state machine defined using a SCXML document, and abstracts out the environment interfaces.
Transaction	Implementations for multi level locks, transactional collections and transactional file access.
Validator	Framework to define validators and validation rules in an xml file.
VFS	Virtual File System component for treating files, FTP, SMB, ZIP and such like as a single logical file system.

http://commons.apache.org/