Portlet Faq   

• Pluto (open source)
• uPortal (open source - http://www.uportal.org/license.html)
• JBoss Portal (open source - LGPL)
• Gridsphere
• Liferay Portal
• eXo Portal
• JetSpeed 1.x (open source)
• JetSpeed 2.x (built on top of Pluto, open source)
• Alfresco (has both commercial and open source versions)
• Oracle Weblogic Portal (closed source)
• IBM Websphere Portal (closed source)
• Vignette Portal (closed source)
• Day Communiqué
• FatWire Content Server

Portlet Repositories

• PortletSwap
• Gems
• Yahoo Group JSR-168/JSR-286
• Portlet Repository
• POST (Portlet Open Source Trading Site)

Resources:

• JSR-168: JCP, Final Spec
• JSR-286: JCP, Final Spec
• Wikipedia: JSR-186 / JSR 286 WSRP
• Portal Admin and jsr168 Portlet Development Tutorials: http://www.portorials.com
• java.net's Portlet Community
• PortletUnit
• Caucho's Portlet Tutorials
• IBM Websphere Portal Zone
• Source code from Portlets in Action book

• Portlets in Action (covers JSR 286)

Articles

• JSR-286: The Edge of Irrelevance
• Hello World Portlet using Spring 3.0 Portlet MVC—Part I
• Spring 3.0 Portlet MVC – Part II (Annotations)