I have heard about Lucene a lot, that it\'s one of the best search engine libraries in Java. Is there any similar (as powerful) library for Ruby?
Well, there's Ferret, which is a port of Lucene to Ruby. Also, Lucene is very easy to use from JRuby, if that's an option for you.
Depending on your needs, you might also want to take a look at Solr, which is a higher-level front-end built on Lucene. There is a Ruby interface, solr-ruby, that interacts with Solr via HTTP.