MySQL’s built-in Full Text Search engine
Open Source Full Text Search engines
A complete list of popular full text search engines is available at WikiMedia site
As a full-text search engine, Lucene needs little introduction. Lucene, an open source project hosted by Apache, aims to produce high-performance full-text indexing and search software. The Java Lucene product itself is a high-performance, high capacity, full-text search tool used by many popular Websites such as the Wikipedia online encyclopedia and TheServerSide.com, as well as in many, many Java applications. It is a fast, reliable tool that has proved its value in countless demanding production environments.
Although Lucene is well known for its full-text indexing, many developers are less aware that it can also provide powerful complementary searching, filtering, and sorting functionalities. Indeed, many searches involve combining full-text searches with filters on different fields or criteria. For example, you may want to search a database of books or articles using a full-text search, but with the possibility to limit the results to certain types of books. Traditionally, this type of criteria-based searching is in the realm of the relational database. However, Lucene offers numerous powerful features that let you efficiently combine full-text searches with criteria-based searches and sorts.
Java World Article on Integrating Lucene