Mammoth PostgreSQL Replicator uses a transaction log and async replication model that nearly eliminates extra load on your master server regardless of the number of slaves you have in production. It supports advanced features such as large object, role, and ACL (grant/revoke) replication. It also supports promotion and failover.
zrep provides an easy-to-use program to manage zfs filesystem replication and failover. No configuration files are required. The program is developed under Solaris, but may work with any up to date zfs implementation. The executable is a single script. Initialization does a full data copy, but subsequent syncs are incrementals. It uses internal locking to make sure there is no danger of overlap if you just shove it in cron to run every minute. Design target is more "near-time replication", since the sync can be run every minute or more. However, it could conceivably be used for "backup" purposes as well.
Skylable SX is a reliable, powerful, fully distributed cluster solution for your data storage needs. It can aggregate the disk space available on multiple servers and merge it into a single storage system. The cluster makes sure that your data is always replicated over multiple nodes (the exact number of copies is defined by the sysadmin) and synchronized. It has built-in support for deduplication, client-side encryption, on-the-fly compression, and much more.