YaCy is decentralized, but without the credit system. Some tokens, like QBUX, have tried to develop decentralized hosting infrastructure.
I also have been wondering how this would play out with some kind of decentralized indexes. The nodes could automatically cluster with other nodes of users sharing the same interests, using some notion of distances between query distributions. The caching and crawling tasks could then be distributed between neighbors.
A big part of the problem I see with decentralized search is that you basically need to traverse the index in orthogonal axes to assemble search results. First you need to search word-wise in order to get result candidates, then sort them rank-wise to get relevant results (this also hinges upon an agreed-upon ranking of domains). That's a damn hard nut to crack for a distributed system.
Crawling is also not as resource consuming as you might think. Sure you can distribute it, but there isn't a huge benefit to this.
I also have been wondering how this would play out with some kind of decentralized indexes. The nodes could automatically cluster with other nodes of users sharing the same interests, using some notion of distances between query distributions. The caching and crawling tasks could then be distributed between neighbors.