Proj.Net

.NET Spatial Reference and Projection Engine
Proj.NET performs point-to-point coordinate conversions between geodetic coordinate systems for use in fx. Geographic Information Systems (GIS) or GPS applications. The spatial reference model used adheres to the Simple Features specification.

• Read the FAQ for common questions.
• Popular Well-Known Text representations for Spatial Reference Systems

Supports:

• Datum transformations
• Geographic, Geocentric, and Projected coordinate systems
• Compatible with both Microsoft .NET 2.0, Mono, .NET Compact Framework & Silverlight
• Converts coordinate systems to/from Well-Known Text (WKT) and to XML

Projection types currently supported:

• Mercator
• Transverse Mercator
• Albers
• Lambert Conformal
• Krovak

See Supported projections for details.

Nuget:
There's also a package available in the Nuget Gallery, created by Mathieu Cartoixa Proj.NET 1.2

Resources:
For an introduction to spatial reference systems see here

If you're working with Google/Bing/OpenLayers, maybe this blog post can help you: The Google Maps / Bing Maps Spherical Mercator Projection