Library not loaded: libpq.5.dylib

For anyone seeing this error and running PostgresApp, this is a known problem that was fixed in version 9.2.4.3:

Discussion: https://github.com/PostgresApp/PostgresApp/issues/109

Installer: https://github.com/PostgresApp/PostgresApp/releases/tag/9.2.4.3

Of course, you could just grab the latest version (http://postgresapp.com/). Both versions got me back up and running with Rails 3.2.14 and Ruby 2.0.0. However, note that when I installed 9.2.4.3 my databases were still listed in psql, but when I installed 9.3.0 (the current version at the moment), my databases were no longer listed.

You may need to remove and re-install the pg gem, so it's compiled against the correct version of Postgres.