Sphinx - Free open-source SQL full-text search engine
"Sphinx is a full-text search engine, distributed under GPL version 2.
[...] Generally, it's a standalone search engine, meant to provide fast, size-efficient and relevant fulltext search functions to other applications. Sphinx was specially designed to integrate well with SQL databases and scripting languages. Currently built-in data sources support fetching data either via direct connection to MySQL or PostgreSQL, or using XML pipe mechanism (a pipe to indexer in special XML-based format which Sphinx recognizes)."
The largest installation has indexed over 1 billion records.