Also, consider DRBD

Sure, you can do that or similar things (loads of SANs have that capability). Keeping the secondary pg instance up2date is really expensive in that scenario though. You need to shutdown pg, drop old, create new snapshot, start pg which will do recovery. If you have nontrivial amounts of writes the wal replay uppon startup will take quite some time....
I don't really see any reason to do so these days unless your database is gigantonormous and you cannot afford to have a full copy of the datadir for reporting.