What's your favorite open source library/project?

We talk about iText, log4j, JFreeChart and JasperReports all the time here, but there are plenty of high quality projects that not everyone may be aware of. I'd like to encourage everybody to list their favorite ones, with a short description of what they like about it, and maybe links to helpful resources (unless they're part of the main project site). I'll go first:

ImageJ - an image processing application and library that can read and write many image formats, and can be extended through a macro language and a plugin API

HSQLDB - all-Java SQL database that installs easily, can run embedded (i.e., no server) and can be accessed by JDBC and Hibernate

Lucene - library that can be used to index and search documents, has powerful search features and is quite fast

http://www.opensymphony.com/compassCompass is a Java Search Engine Framework built on top of Lucene. It provides simplified ORM like API when working with a Search Engine and extending Lucene supporting transactional index operations.

OSCache

OSCache is a caching solution that includes a JSP tag library and set of classes to perform fine grained dynamic caching of JSP content, servlet responses or arbitrary objects.

SiteMesh

SiteMesh is a web-page layout and decoration framework and web- application integration framework to aid in creating large sites consisting of many pages for which a consistent look/feel, navigation and layout scheme is required.

Quartz

Quartz is a full-featured, open source job scheduling system that can be integrated with, or used along side virtually any J2EE or J2SE application.

ClickStream

Utility to track the current users on your site, and where they've been in detail.
(Description is from OpenSymphony.com)
[ February 18, 2007: Message edited by: John Todd ]

POI-HSSF project is meant for Excel file operations. Started using it just a couple of months back. Looks good. They even have libraries for other Microsoft Format Files

DocBook XML and DocBook XSL for creating and versioning of custom PDF, HTML and JavaHelp documents

Bump. Nobody else has anything to share?

Java CSV A simple library to read/write csv files

Umlet A no-fluff tool to make UML diagrams in no time. (needs a bit of practice though)

JTDS Throw away your crappy Microsoft driver and get this JDBC Driver for SQLServer (and Sybase).

lint4j Analyze your source and spot some dirty code

Originally posted by Satou kurinosuke:

lint4j Analyze your source and spot some dirty code

This looks like a good one, going by what has been mentioned on their home page.

JForms - Swing GUI Framework from JGoodies

Whether lint4j detects memory leak too ?

Whether lint4j detects memory leak too ?

No. It's a static code checker, and consequently does nothing at runtime.

jDepend by Mike Clark
xStream by xstream.codehaus.org
JavaLayer by http://www.javazoom.net/javalayer/javalayer.html

Joda-Time: each time I see a question on Date or Calendar pop up, I'm happy I discovered Joda-Time.

Lucene. It has to be one of the best pieces of openSource software I've come across. Optimization is somewhat inefficient, but on the whole, itts rock-solid.

POI-HSSF project is meant for Excel file operations.

I am using it from 2 months... its very good.... GO for it...
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Checkstyle is a tool that provides an automated and fully configurable way of checking your coding style. It has en eclipse-plugin too. When you're working in a team, it's a good thing to define a coding style and when using Eclipse it immediately says if your code is not addhering to it. Some examples of what can it check are: using brackets in same or new line, not having defined Javadoc for public and protected methos, variable names styles (like final static variables being in uppercase), have a new line at the end of a file, and so on...

By the way, I've just looked at eclipse-cs site and found they received an award:

The Eclipse Checkstyle plug-in won the Eclipse Community Award 2007 in the category of "Best Open Source Eclipse-based Developer tool".

[ March 08, 2007: Message edited by: Joan Horta Tosas ]

I love the apache commons.

Web-Harvest

Sweetdev Ria Ajax library for J2ee.

Ananth Chellathurai