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."