I might be behind the curve on this one, because I didn’t even know YaCy (a p2p search engine) existed, let alone that you could hack/customize it! The concept immediately resonated with me:

One of the most exciting things about YaCy … is the fact that it’s a local client. Each user owns and operates a node in a globally distributed search engine infrastructure, which means each user is in full control of how they navigate and experience the World Wide Web.