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.
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.
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 ]