One of the most exciting projects around is CouchDB. We've been playing with it for months, and we even used it in Speeqe for group chat logging. It is built like the Web; it scales like the Web; and it's as cool as the Web.
It replicates well in any topology you desire. Finally, we have master-master replication that works well. It can do this because it doesn't assume away the problem of data conflicts. Instead, CouchDB provides tools to deal with conflicts.
Still not convinced? CouchDB is a peer-to-peer to application server out of the box, thanks to its well done Web interface. It also has the coolest logo of any project I've seen in quite some time.
So relax, and check out CouchDB.