postgreSQL hot_standby pg_dump on slave
Just run pg_dump.  The documentation says:


  pg_dump is a utility for backing up a PostgreSQL database. It makes
  consistent backups even if the database is being used concurrently.
  pg_dump does not block other users accessing the database (readers
  or writers).


In turn, the state of the database that you see on the slave is also consistent.  If there is a considerable delay in replication, the result may be slightly different than a dump from the master.