http://www.arakoon.org uses (Multi-)Paxos. Version 2.0 in development at https://github.com/Incubaid/arakoon (e.g. see https://github.com/Incubaid/arakoon/blob/2.0/src/hope/mp.ml for the state machine implementation).

https://news.ycombinator.com/item?id=4052123 ?

I think Apache Zookeeper is a Paxos implementation. I've heard very good things about it. Various pieces of Hadoop use it, and I know of people who use it for medium-sized webapps that need a canonical, fault-tolerant configuration system.