The dotSesame project is a C# port of the Sesame project, which is originally written in Java. It is an open source RDF database with support for RDF Schema inferencing and querying. Full documentation of Sesame can be found at http://www.openrdf.org/
I like it. I have posted it on my blog. 10x!
Using this .Net port of the Sesame framework in several projects, even in production environments, and it is both stable and fast. Ideal solution for use of a good RDF API and SPARQL engine in .Net surroundings.