You'll want to check out the
clustering systems, anything stating to provide write-scaling is probably multi-master.
PG-XC describes itself as a "write-scalable synchronous multi-master postgresql cluster". Interestingly 'synchronous' implies that this system scales out without sacrificing consistency. I haven't used it so I can't say for sure. My experience is with commercial RDBMSs and the eventual consistency model.
Data & Reality looks like an interesting read for sure, I will add it to my queue thanks!