A db is either centralized with a master server or not. These definitely work without a master.
The tagline at the github repo at https://github.com/basho/riak is "Riak is a decentralized datastore from Basho Technologies". I don't think they would lie.
By decentralization they mean decentralization of control, which is not the same thing as in Riak, where control is very much centralized. Think distributed hash tables, like in bittorrent or, obviously, bitcoin.