The three most popular ones are:
ferret (port of lucene) solr (really is lucene) sphinx
For sphinx, you'll want to look at http://seattlerb.org/Sphincter
which is the "Sphincter" plugin. Don't ask me, I didn't name it :).
Here's an article on the acts_as_ferret plugin: http:// www.railsenvy.com/2007/2/19/acts-as-ferret-tutorial Here's the acts_as_ferret Trac page: http://projects.jkraemer.net/ acts_as_ferret/wiki
For solr, check out acts_as_solr: http://acts-as-solr.rubyforge.org/
Each has its strengths and weaknesses. YMMV.
Good luck!