I was looking at Lucene .NET for a full-text search engine implementation but I was surprised to find out that the Lucene.NET application library is not open source anymore. It has been relaunched as a commercial product called SearchBlackBox. Lucene .NET is the core search technology behind the popular Lookout plugin for Outlook (later bought by Microsoft). Too bad that such a cool technology is not open source anymore. Atleast the developers could have left the last stable release of Lucene .NET under a public license. I'm not sure if there's any other open source search library for .NET in the market.