Confluent's Apache Kafka .NET client
A distributed transaction framework that supports multiple languages
ECG Toolkit support for: SCP-ECG, DICOM, HL7 aECG, ISHNE & MUSE-XML
Cross-platform DICOM implementation
C# Extension Methods | Over 1000 extension methods:
Apache Cayenne, Wicket, CXF, GWT, EJB examples as a framework