Here is my short list of the most important open source [free] information retrieval libraries being used today that are undergoing active development as of writing. It is a mixture of engines used for both research and industrial applications.
Wikia is working to develop and popularize a freely licensed (open source) search engine. What you see here is our first alpha release.
recherche mit tags, synonym und token in flash
open source code suchmaschine, programmiersprache vorauswählbar