This article was first written in March 2005 for the BeezNest technical website (http://glasnost.beeznest.org/articles/215).
Probably the best reference about Searching Engines: http://www.searchtools.com/tools/
Of special interest in the Open Source market:
- Apache Lucene
- Hyper Estraier
- SPyRO, SPyDI, and related Projects