MyEclipse For J2EE development. Note: I primarily use it as a JSP, Javascript, CSS, TLD, DTD editor
XMLBuddy For XML file editing.
Colorer Basically a general-purpose syntax highlighter (supporting bazillion formats and languages)
GEF (Graphical Editing Framework). Pre-requisite for Spring UI plugin (see the next entry).
Jalopy (commercial, latest stable version: 1.2)
RegEx Tester Install it if you're planning on using Regular expressions a lot in your development.
Spring UI plugin. URL: http://www.springframework.org/spring-ide/eclipse/
Easy Explorer. URL: http://easystruts.sourceforge.net/eclipse/updates/site.xml
VeloEclipse. URL: http://propsorter.sourceforge.net/veloeclipse
Hibernate Synchronizer URL: http://www.binamics.com/hibernatesync
Log4E URL: http://log4e.jayefem.de/update
PS:
For quick access to Windows explorer from within Eclipse. Install it using Eclipse's Update Manager: Help -> Software Updates -> Find and Install... -> Search for new features to install -> Add Update Site...