Itinero is a routing project for .NET/Mono to calculate routes in a road network. By default the routing network is based on OpenStreetMap (OSM) but it's possible to load any road network. The most important features:

• Calculating routes from A->B.
• Calculating distance/time matrices between a set of locations.
• Processing OSM-data into a routable network.
• Generating turn-by-turn instructions.
• Routing on mobile devices and lower-resource environments.

Check the wiki for documentation and sample code. Don't hesitate to ask questions using an issue or request more documentation on any topic you need.