SearchWP

SearchWP works basically out of the box. The key advantage of this over other options is the ability to pull in custom fields and adjust weighting across custom fields, titles, content, and taxonomies. Although the speed of the engine depends on the ability of your server, it is much cheaper than using a hosted search option. Support is caring and helpful. I'm looking forward to upgrading to version 4.0, since it is supposed to be faster. I hope, too, that this new version will support growth of the site beyond the current ceiling of approximately 20,000 post items.