DotNetNuke
Open Source Framework ideal for creating Enterprise Web Applications
I'm working on a project with friends and for this project we needed a Free Web Application Framework that will support Forums, Blogs, Shopping Cart, BackOffice and the most important is a Framework that can be modified easily and quickly.
DotNetNuke is one of the Top Web Application Framework on the market and it's Free.
If you are in the Web Business I recommend reading about DotNetNuke!
Web Application Framework
DotNetNuke is an open-source Web Application Framework ideal for creating and deploying projects such as commercial websites, corporate intranets and extranets, online publishing portals, and custom vertical applications.
DotNetNuke is provided as open-source software, licensed under a BSD agreement. In general, this license grants the general public permission to obtain the software free-of-charge. It also allows individuals to do whatever they wish with the application framework, both commercially and non-commercially, with the simple requirement of giving credit back to the DotNetNuke project community.
DotNetNuke is built on a Microsoft ASP.NET (VB.NET) platform, and is easily installed and hosted. With a growing community of over 440,000 users, and a dedicated base of programming professionals, support for DotNetNuke is always close at hand. 
Here you can watch a Online Demo
http://blogs.microsoft.co.il/blogs/shair/archive/2008/08/27/dotnetnuke.aspx
Web Application Framework
Here you can watch a Online Demo
