(Wrote this a few weeks ago when I knew nothing. Indented into my brain now. Should have published earlier or just trashed the post as it seems too simple now. So instead I'll update it when I find out some new neat tricks.)
List of databases: $ psql -l
To open one of them, $ psql MyDatabase
To see what is in the database (list of relations), mydb=# \d
To examine a specific table, mydb=# \d TableName
Can execute SQL, mydb=# select * from Table;
Can edit SQL in an editor from within PSQL, mydb=# \e
To quit, mydb=# \q
To load a schema from a file $ psql mydb -f /home/foo/bar
Also from the shell, $ pg_dump -O myDB > file (-O means no ownership information is outputed)
On my server configuration (default for ubuntu) you can restart the PostgreSQL service using, $ sudo /etc/init.d/postgresql-8.3 restart