NetTopologySuite
A .NET GIS solution that is fast and reliable for the .NET platform.
NetTopologySuite is a direct-port of all the functionalities offered by JTS Topology Suite: NTS expose JTS in a '.NET way', as example using Properties, Indexers etc...
An excerpt from JTS website explains the capabilities of NTS too:
"The JTS Topology Suite is an API for modelling and manipulating 2-dimensional linear geometry. It provides numerous geometric predicates and functions. JTS conforms to the Simple Features Specification for SQL published by the Open GIS Consortium."
Enjoy using them!
Install with NuGet package manager 
PM> Install-Package NetTopologySuite
Clone
When cloning make sure you also clone the submodules:
git clone --recursive https://github.com/NetTopologySuite/NetTopologySuite