oss4lib

As seen at Rubyforge: Blacklight is "a next generation library catalog written in ruby, using solr as the underlying search engine. All you have to do is export your marc records, index them with the scripts provided, start up ruby on rails, and you're on your way to faceted browsing bliss."

Links to a demo, docs, and the project list are all available at blacklight.rubyforge.org.