This important page has inormation on how to buy bus tickets, how to get to the ports, and how to get to different places in and around Athens using public transport including buses, trams, trolley, the Metro and ISAP among others
For more info about the buses see buses, and the same goes for tram and Athens Metro
GNTO gives out a really great pocket map of Athens Public Transport. You can get it from their office on Amalias Street. Thats where I get this printout too.
Athens Practical Info Index