DotNetNuke® (also known as DNN) is a free, Open Source Framework ideal for creating Enterprise Web Applications. 

The advantages of DotNetNuke® as a web solution, in my opinion, are: 

It's scalable. DotNetNuke® comes with core functionality, but if you find you need additional functionality then the core modules or additional modules may be developed specifically for you, or indeed the module may already be available from the DotNetNuke® community. If you have modest requirements then the cost is usually not exhorbitant. 

Design is seperated from the back-end, so you can change the look for your site (called the Skin) easily and more importantly, cost effectively. 

DotNetNuke® is Open Source and free to install, making it a cost effective solution. 

It's built on a well known architecture, asp.net, Microsoft SQL & IIS. There are lots of developers out there with these skills, so you can shop around for the best solution at a cost that suits your pocket. Alternatively, you can choose to skill up yourself; there is plenty of free tutorials on the Net and in addition as DNN is Open Source you have access to the source code. 

Last and importantly, DotNetNuke® is an end-user friendly Website Content Management System i.e. with training your end users can add, edit, delete sections, pages and content on their website without any input from IT.