This explains how to create an external data wrapper.
So, how to list all existing external data wrappers?
Many FDWs are listed on the Postgres Wiki:
https://wiki.postgresql.org/wiki/Foreign_data_wrappers
psqlhas useful commands, and also know which FDW are defined, for example. \dew[+]:
psql
\dew[+]
http://www.postgresql.org/docs/current/static/app-psql.html
Just \detor \det[+]for more details.
\det
\det[+]
See the official documentation here