Fossil is primarily a DVCS, like git, but aimed at a different set of users (e.g. smaller projects) and includes decentralised tickets etc., but in practice it is easiest to have a centralised server. Its easy to run your own, but there is hosting available for Fossil: https://chiselapp.com/

Redicale seems much more decentralised. Rather than making it easy to run your own server it eliminates the server altogether. A more radical approach :).

it would probably be easier to build something like Radicale on top of Fossil because all that is missing is a way of finding to peers, and connecting to them without a server (e.g. from dynamic IPs, behind NAT etc.).