Sphinx search (www.sphinxsearch.com) is one of the fastest developing search engine. Sphinx is used by Craigslist amongst several popular sites. If you have not already tried Sphinx and you are interested in a building your own search engine, give it a shot. Some of the salient features of sphinx is around easier integration with MySQL, fast indexing and support for distributed searches.
Support Sphinx and nominate it for the community choice awards!