The distributed search model that YaCy uses would never work in a large scale enterprise. Security, safe harbor, etc are all difficult enough using a traditional, centralized approach. Trying to imagine this done in a distributed way across the enterprise is giving me a headache.

And the closest thing to open source, turnkey search is gluing together Apache SOLR and a web crawler. Lucid Imagination offers this (plus other features) as a commercial product, but it not open source to the best of my knowledge.

I was playing with YaCy a little, and there is an "Intranet" mode. As far as I can make out, this can operate in a distributed way, but behind the firewall. I didn't look into how to set it up in great detail yet though, was playing with web search.