Apache Solr is an excellent full text index search engine based on Lucene. Solr is increasingly being used in the Drupal community for search. I use it for search for a lot of my projects. Recently Steve Edwards at Drupal Connect blogged about setting up a mutli core Solr server on Ubuntu 9.10 (aka Karmic). Ubuntu 10.04LTS was released a couple of months ago and it makes the process a bit easier, as Apache Solr 1.