I recently spent some time searching for a good open-source .NET HL7 parser. The pickings were pretty slim, and the best option appears to be NHapi--which is a fairly new port of the popular Hapi Java HL7 parser. I had to dig surprising deep into the search results to find NHapi, and though the code seems solid, the project is not extremely active. Another big weakness is the complete lack of API documentation and code samples. You can figure out the essentials from the Hapi Javadoc and sample code, but that's a bit of a pain so I've generated MS Help API docs for each HL7 version supported by NHapi. They are available in a single zip file here (80MB).