There are quite a few search-related contributed modules on drupal.org -- you can see the entire list (unfortunately in date order, or perhaps some random order, rather than alphabetical order) here: http://drupal.org/taxonomy/term/105
A few of the search-related modules have handbook pages: http://drupal.org/node/228411
If you want to compare different search methods, there are several outside resources you may find helpful:
- An article that compares different methods for searching in Drupal 6: http://poplarware.com/drupalsearch.html
- A DrupalCon DC 2009 session comparing several search methods (with video): http://dc2009.drupalcon.org/session/lightning-panel-search-drupal.html
- A DrupalCon Szeged 2008 session comparing several search methods (with video and slides from the talk): http://szeged2008.drupalcon.org/program/sessions/drupal-search-where-are...