YaCy is an open source peer-to-peer search engine that has been flying under most of our radars for the last 10 years. After looking at it for a couple of days I’ve begun to see the possibilities.
YaCy is written in Java and uses a constellation of open source technologies to work its magic.
One of its potential applications of interest to me would be using it in a standalone private search appliance on a corporate intranet, much as some companies today use a Google Search Appliance. Other use cases can be found on the YaCy Wiki Use Cases page.