select datname from pg_database;

Others to try:

SELECT *FROM information_schema.columns
SELECT distinct table_catalog, table_schema,table_name, column_name FROM information_schema.columns 


using pgAdmin III

Note: In pgAdmin III, you must select a Database to enable SQL Tool:

I have two Users/Owners, default is postgres.

In the query below I used postgres, default user's SQL worksheet. Both the users are allowed to access all the databases in PostgreSQL dbms