free oss – comes with .net libs and single dll embedded version
Firebird is a relational database offering many ANSI SQL standard features that runs on Linux, Windows, and a variety of Unix platforms. Firebird offers excellent concurrency, high performan

Lucene.Net is a source code, class-per-class, API-per-API and algorithmatic port of the Java Lucene search engine to the C# and .NET platform utilizing Microsoft .NET Framework.
Joel Spolsky says this is what the excellent lookout was built on top of.