pg_controldata from SQL
Like many database, PostgreSQL stores critical (minimal) state about the database in what is called a "control file." This control file has valuable information in it that speaks to backups, checkpoints, block sizes, etc. PostgreSQL ships a tool called pg_controldata to dump this file's values in human-readable form. I've been frustrated in the past that you can't see all these values from within a PostgreSQL SQL session. At some point in the past I got in an argument about the usefulness of such a feature and I pretty well lost that argument: a postgres control file on an active database doesn't really show you (much) useful information and you really need it when the database is off (which is what pg_controldata provides).
PostgreSQL 9.0 changes the game. You can run queries on a database that isn't active (particularly a standby database that is applying WAL files). Now this feature becomes much more interesting. I can use monitoring tools with SQL-only access to find out extremely useful things about the state of the standby.