If you're looking to search an existing MYSQL DB I hear Sphinx is a good option. I personally use Ferret but everyone seems to think it sucks horribly so I'm sure you'll hear that from people. I have had no problems with Ferret at all however. Sphinx and Ferret are both Ruby based.

Solr is also an option but it tends to be slower than Ferret and require more resources.

Here is a recent post of mine that gives a quick comparison of Ferret and Solr. http://www.embought.com/blog/show/10?t=SOLR-vs-Ferret-as-a-S...