DotNetNuke is one of the largest open source portal frameworks available.  We have used this portal framework to develop solutions for sites of all sizes and we highly recommend this platform for the development of web based applications because of both its maturity and its extensibility.  While DotNetNuke provides a full-featured portal framework, it's also a great platform for developing web applications, especially applications which will require community features.  The size of the community is one of the greatest advantages of this platform.  Solutions built using the DotNetNuke infrastructure can be maintained by any of the thousands of DotNetNuke developers who are actively contributing to the platform.  Contact Us today if you're interested in knowing more about how DotNetNuke can help save your organization thousands of dollars and provide you with a rich feature set that will rival even the best commercial CMS solutions available.  Our clients, including national associations, have told us after several years of managing their entire web presence using the DotNetNuke infrastructure that they're still very happy with the product.