DNN, formerly DotNetNuke is a Content Management System built for the Microsoft's .NET Framework. First released in 2003, and currently on version 9, DNN powers over 1 million websites. While that's small in comparison to some of the other CMSes, it is by far the largest available on the .NET Framework.
Plugins and Themes
DNN has a fair amount of plugins and themes, nowhere near the number you'll find on WordPress of course, but there isn't so much redundancy as you'll find in the WordPress ecosystem. For most functionality, it should not be too challenging to find a plugin to fill your needs.
DNN has been built with a strong focus on security. According to their marketing material, the Department of Defense runs hundreds of sites on DNN. Security being one of the weaknesses of the higher profile CMSes, if this is one of your concerns or requirements, DNN may be worthy of consideration.
If your technology stack is already decided and you're running on the .NET Framework, then DNN is going to be one of your main contenders. The .NET Framework is being sidelined by Microsoft in favour of it's cross platform .NET Core. There is talk of migrating DNN over to .NET Core, although given how extensive the rework will need to be (not to mention the fact that all plugins and themes are built with .NET Framework in mind, so will also need to be rebuilt), it's unlikely that this is something that will happen soon, if ever. The .NET Framework will be supported by Microsoft until at least 2029, so this is not seen as a major problem at the moment, however, given the cross platform nature of .NET Core, a potential migration may see DNN reaching a wider audience.