If you're building a small-scale, low-cost enterprise portal for sharing information and documents, take a look at Liferay Portal. Liferay Portal is a free open source enterprise portal with many collaborative features, such as:
-- Web Content Management, which enables you to edit portal Web pages in a WYSIWYG manner
-- Document Library for sharing files and documents
-- Calendar for scheduling meetings and events
-- Message Board for threaded discussions
-- Blogs, Wikis and Polls
-- And many more features: click here to read
Liferay Portal is developed in Java and works with Windows and Linux. I was able to get the portal running in a few minutes just by downloading the software here (I used the Tomcat 6 bundle) and following the installation instructions here.
The portal is bundled with the free HSQLDB database, which is suitable only for very simple portals. If you plan to store lots of data in the portal, you should migrate to a better database like MySQL.
Liferay Portal comes in two editions: Standard and Enterprise Editions. The Standard Edition is available at no cost, and comes without technical support. If you prefer to have technical support, especially if your portal is large and needs to be secure, consider buying the Enterprise Edition.