• MyEclipse  For J2EE development. Note: I primarily use it as a JSP, Javascript, CSS, TLD, DTD editor
• XMLBuddy For XML file editing.
• Jalopy (commercial, latest stable version: 1.2)
• RegEx Tester Install it if you're planning on using Regular expressions a lot in your development.

• Easy Struts  URL:  http://easystruts.sourceforge.net/eclipse/updates/site.xml
• Spring UI plugin. URL: http://www.springframework.org/spring-ide/eclipse/
• Easy Explorer. URL: http://easystruts.sourceforge.net/eclipse/updates/site.xml
• Velocity.  URL: http://propsorter.sourceforge.net/veloeclipse
• FreeMarker URL: http://www.freemarker.org/eclipse/update 
• JBoss-IDE  URL: http://jboss.sourceforge.net/jbosside/updates
• Hibernate Synchronizer URL:  http://www.binamics.com/hibernatesync
• Python URL: http://pydev.sourceforge.net/updates/
• Perl URL: http://e-p-i-c.sourceforge.net/updates
• Properties Editor  URL:http://propedit.sourceforge.jp/eclipse_plugins/3/
• Log4E   URL:  http://log4e.jayefem.de/update
• Spindle URL: http://spindle.sourceforge.net/updates
• Tapestry Palette  URL:http://www.mjhenderson.com/updates

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...