Sphinx Search, an open source tool for indexing and searching data. Works well with MySQL.
Be sure to use the debug version (./configure –with-debug) when you first start evaluating it. What might turn into silent corruption in the production version is likely to crash in debug.
It uses crc32 values instead of storing the actual words in its index. Once you get over a few hundred thousand words, collisions become likely. That can lead to time wasted debugging weird query results where you get back apparently unrelated matches.
