PostgreSQL "freeze"/"unfreeze" command equivalents
There's no exact equivalent to what you want. Options include:

Atomic file-system snapshot

If you're using an atomic file system snapshot you don't need to freeze the database. It might make recovery a little faster if you force a CHECKPOINT first, but that's about it. When you take a filesystem snapshot and copy it, then start the copy, to PostgreSQL it's as if it crashed and is starting back up. That's perfectly fine, it's crash safe and designed to work that way.

This approach is only valid if the snapshot is atomic - it's all at the same virtual instant. You can't get an atomic snapshot across multiple file systems (on Linux, at least), so if your DB is split across multiple tablespaces or WAL is on a separate disk to the heap you cannot use this approach.

pg_start_backup / pg_stop_backup

If you can't do an atomic snapshot you can enable WAL archiving, run pg_start_backup, copy the DB, run pg_stop_backup and capture the last WAL archives generated.

It's a bit more complicated, but it gives you a consistent backup without stopping writes and without needing file-system level atomic snapshots.

pg_basebackup

An alternative to using pg_start_backup and pg_stop_backup is to use pg_basebackup to do a streaming copy of the DB over the PostgreSQL replication protocol with --xlog-method=stream. This requires only a PostgreSQL replication connection, doesn't require the DB to be stopped, and is pretty seamless.

--xlog-method=stream was only added in pretty recent versions, and pg_basebackup its self is fairly new.

pg_dump

I didn't initially mention it because you were looking for external tools, but there's always pg_dump, which gets a SERIALIZABLE snapshot of the database and dumps it. The DB keeps running like normal (it can still accept writes) and the dump is entirely internally consistent from the time you started the dump.

Write quiescence

Stopping all incoming transactions won't stop PostgreSQL writing. It'll still have VACUUM work to do with autovacuum, checkpoints to perform, stats to write, etc. 

There's no feature in Pg to stop all writes at this point. It might be nice to add, but I'm not aware of anyone working on it. 

Some file systems, like XFS, support write freezing at the file system level; this causes all writes to block until the freeze is released. It's safe to freeze all file systems then copy all file systems.