Standards support is an important criterion for most corporate development projects. In the area of enterprise portal servers, that means a J2EE-compliant engine that supports standards such as portlets (JSR 168) and WSRP (Web Services for Remote Portlets). Fortunately, there are quite a number of open source projects competing in this space.
One standout is the eXo platform. Already in its production phase, this project is designed as a customizable enterprise portal and CMS. Its portlet container is actually certified compliant with JSR 168 and has been enhanced with caching, connection pooling, and shared-session features. The portal itself is based on JavaServer Faces (JSR 127) technology and offers a Model View Controller architecture. The software is provided under a dual license -- both commercial and GPL -- and the eXo project team can provide both online support and on-site training for a fee.
Another contender is Liferay Portal, which sports a feature set similar to that of eXo, including JSR 168 and WSRP compliance. Instead of JavaServer Faces, however, Liferay uses the more mature Struts and Tiles technologies. It runs atop a wide variety of application servers and databases and provides hooks for SSO (single sign-on) capabilities, although only Yale University's SSO engine is supported out of the box. As with eXo, professional support is available from the Liferay developers themselves, although the support costs can quickly add up. The software itself is available under the MIT license, which is similar to the Apache license.
The Apache Software Foundation has long been interested in portal software and is currently home to a number of projects in this area. Jetspeed-1 software is a basic portal system built on technologies such as JSP and XML. Jetspeed-2 is more component-based and offers a more modern, portlet-based approach. Apache also offers a reference implementation of a portlet container, called Pluto. All are available under the Apache license.
JBoss has also thrown its hat into the ring with JBoss Portal, a part of the company's JEMS (JBoss Enterprise Middleware System) Java application development stack. This is another JSR 168-compliant portlet container that offers customization through JavaServer Faces, MyFaces, and Spring, plus additional content management and administration. As "the professional open source company," JBoss offers both GNU LGPL (Lesser General Public License) and commercial licensing, in addition to professional support, consulting, and training.
Finally, GridSphere is another JSR 168-compliant portlet-based portal server with support for JavaServer Faces. One notable extra is an additional portlet API that claims almost 100 percent IBM WebSphere compatibility. Consulting, training, and development support are available through Gridwise Technologies. The software is provided under an independent, non-OSI-approved GridSphere license, however.
This list gives you a number of options, and still other choices are available. Determining which project's offering works best for you will doubtless be a process of experimentation.