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. SearchBlackBox for .NET, is the full-text search engine library for .NET compatible with Lucene.Net and Jakarta Lucene 1.4-final. SearchBlackBox allows you to add full-text searching capabilities to all kind of applications from web site search to enterprise search applications. SearchBlackBox is compatible with .NET 1.x, Mono and Jakarta Lucene.