Eclipse is a kind of universal tool platform - an open extensible IDE for anything and nothing in particular.

Eclipse Homepage

We recommend that you download the WTP (Web Tools Platform) version which contains an excellent XML editor as well as other tools. Download from:

http://download.eclipse.org/webtools/downloads/

• ASDT - Actionscript perspective and editor, which can call mtasc.
• FlashOut - Log output view.
• Flair - A Set of plugins for log and view SWFs
• IDE for Laszlo - IBM's Eclipse plug-in for OpenLaszlo

• Eclipse Plugins - Directory listing loads of plugins here.
• PHPEclipse
• SubEclipse - A Subversion/Eclipse plugin
• TortoiseSVN - TortoiseSVN is a SubVersion client
• SVAnt - An Ant task that provides access to Subversion from Ant scripts
• VSS plugin - A Visual SourceSafe plugin
• EclipseUML/Omondo - UML editor
• CFEclipse - CFML editor plugin
• Eclipse-games - Eclipse-games
• Eclipse RSS Reader - Eclipse RSS Reader
• JOPE - JOnAS Plugin for Eclipse

Martin Wood posted a guide to creating a simple plugin for eclipse on the osflash mailing list. It can be found here: Plugin Development Guide

Eclipse Tips & Tricks section on OSFlash