Ferret is fast. He has moved around and the documentation is a bit sparse.
Solr was created by people at CNET, used by Internet Archive, Krugle, Revolution Health, his projects: Collext, Blacklight.
Findability – serendipity is great. You should be able to browse and discover stuff you didn’t intend to.
Read Ambient Findability
Lucene is fast and scalable. Written in Java.
Solr is a layer on top of Lucene.
Flare is a plugin he wrote that is a bit of a hack but does “faceted browsing, auto-suggest, folksonomy tagging/annotating.”
Adds caching, replication, faceting, highlighting, spell checking, Http interface.
You can talk to it via Net::HTTP::Post or use solr-ruby (gem), acts_as_solr (will hopefully use gem soon, Hatcher submitted a patch)
gem install solr-ruby
Look at the code in his slides.
Luke is a tool that lets you view your lucene index.
This is a pretty sweet tool for adding search to your site. I plan on looking into it further.