Eclipse + plug-in

eclipse plug-in
http://www.eclipse.org/articles/Article-Plug-in-architecture/plugin_architecture.html

http://www.numeclipse.org/architecture.html

Crawljax is an open source Java tool for automatically crawling and testing modern (Ajax) web applications.
http://crawljax.com/

Wicked Shell is an Eclipse plugin providing direct access to your system's shell.
http://www.wickedshell.net/

Eclipse + GDB
http://wiki.eclipse.org/DSDP/DD/GDB

http://code.google.com/p/tarlog-plugins/

http://explorerplugin.sourceforge.net/