Solr is a blazing fast open source enterprise search platform. It is based on the the Apache Lucene project. It allows for powerful full-text searching, hit highlighting, faceted search, rich document (e.g., Word, PDF) handling, and geospatial search. Solr is highly scalable and known to handle millions of indexes, across a distributed environment.

It powers the search and navigation features of many of the world's largest internet sites. Some major sites that use SOLR are:

Here's a presentation that would be useful in making a choice to enable powerful searching using SOLR for your Drupal websites.

search_apache_solr_drupal.pdf (202 downloads)