Definition
In general, open source refers to any program whose source code is made available for use or modification as users or other developers see fit. Open source software is usually developed as a public collaboration between communities of developers and made freely available.
There's no such thing as a free lunch...
Well, this might be one of the exceptions to that old saying. However, increasingly developers are including their Open-Source activities on their CVs as proof that they are passionate about what they do, to the extent of giving up free time to collaborate with others. Of course, they also benefit because they don't always have to 'invent the wheel' when programming - someone, somewhere may have done it already, and is willing to share!
DotNetNuke
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 200,000 users, and a dedicated base of programming professionals, support for DotNetNuke is always close at hand.