Real world usage of concatenative programming languages

As Greg wrote, postscript is the mammoth example.

Concatenative languages pop up everywhere, quite naturally, because of the trivial nature of the language runtime. It's a favourite for many firmwares: I first encountered Forth "in the flesh" in the bootloader for a Sun Sparcstation. It powers the firmware for the OLPC.

Ocaml's parent, Caml was based on realising the semantics of functional programming as the Categorical Abstract Machine (the CAM in Caml).

Bibtex uses a concatenative language to compile style files.

Take a look at FORTH Inc, They list several projects that they and their customers did, using their FORTH.

Part of the firmware on Macs (at least in the older PowerPC models) was written in Forth.

See: Link

Eserv and nncron are written in SP-Forth.