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Components
On this wiki:

BeanUtils - Utilities for manipulating java beans, including copying properties, cloning, invoking methods and type conversion.

Betwixt - Services for mapping JavaBeans to XML documents, and vice versa.

Chain - A "Chain of Responsibility" pattern implemention for organizing complex processing flows.

CLI - Command Line Interface library for processing command-line options and arguments.

Codec - General encoding/decoding algorithms. Includes some phonetic encoders, Hex, Base64, and a URL encoder.

Collections - Builds upon the Java Collections Framework of List, Set and Map to provide many more implementations, new collections and abstract base classes.

Configuration - Tools to assist in the reading of configuration/preferences files in various formats

Daemon - Run (java) applications as windows service or UNIX daemon.

DBCP - Database connection pooling services.

Digester - An XML-to-Java-object mapping utility commonly used for parsing XML configuration files.

Email - Simplify email sending with Java. Built on top of the Java Mail API.

FileUpload - Makes it easy to add robust, high-performance, file upload capability to your servlets and web applications

HttpClient - A framework for working with the client-side of the HTTP protocol.

IO - Utility classes, stream implementations, file filters and endian classes to assist in input/output operations.

JCI - Java Compiler Interface

JEXL - A velocity-esque expression interpreter

Jelly - An XML-based scripting language

JXPath - Facilities for applying xpath expressions to graphs of objects of all kinds: Java beans, Maps, Servlet contexts, DOM etc.

Lang - Common utilities that should be provided by the JDK, including String handling, Object and Date helpers, nested exceptions and enumerated types.

Launcher - A cross platform Java application launcher which eliminates the need for a batch or shell script to launch a Java class.

Logging A thin, modular bridging API with out-of-the-box support for the most well known logging systems.

Math Lightweight, self-contained mathematics and statistics components.

Modeler Mechanisms to create Model MBeans compatible with the Java Management Extensions (JMX) specification.

Net - A collection of classes implementing various network protocols such as FTP, NNTP, SMTP, Telnet.

Pool - A generic object pooling interface, a toolkit for creating modular object pools and several general purpose pool implementations.

SCXML - A Java implementation of State Chart XML (SCXML), currently a W3C Working Draft.

Transaction - Utility classes commonly used in transactional programming

Validator - Components for user input validation

VFS - Extensible framework to acces various filesystems through an unified api with implementations for e.g. sftp, smb, http, zip, ...

On the commons site:

http://commons.apache.org/components.html