The best search engines on the internet will depend on the kind of information the user is looking for though there are some that are accepted by most users as being helpful such as; Google, Abilock's article, Metacrawler, AltaVista and Profusion.
If your questions mean something like 'I need search capabilities in my .NET application, can I use some existing code? then I guess. Lucene.NET. can be helpful, although I wanna read fist this stuff: http://goo.gl/8ME2K. www.quora.com