PaxosPaxos is an algorithm that can be used for active-active replication between multiple masters. In the (patented) version we use, it has been enhanced to work well in a WAN environment.
A nice feature is the entirely of failure conditions are contained within the algorithm, so the issues of data loss or packet loss are not a threat to data safety. The amount of data you can afford to lose can be zero. (Assuming the entire universe isn't blown up at once.)