Athens Bus Network (http://www.oasa.gr)
A very useful site for Athens Public Buses services.
Lots of information concerning bus routes, terminals, schedules etc.
You can even download a complete map of Athens with the bus network
(enormous PDF files, only for very fast connections)

Attiko Metro (http://www.ametro.gr/)
The site of "Attiko Metro" gives useful information for the subway network of Athens. 

I.S.A.P.  (http://www.isap.gr/index.asp)
The "Ilektrikos" is the first subway line of Athens in service since 1869.
It links the Piraeus harbour with Athens and the northern suburbs.
Since it's operated by a different consortium than "Attiko Metro" it has a different Web Site... 

Tram (http://www.tramsa.gr/)
The new way of transportation in Athens with lines that connect downtown with the coast. We are located on “Pikrodafni” station.

El. Venizelos Airport (http://www.aia.gr/)
The official Athens international Airport site.

Intercity Bus System (http://www.ktel.org/)
To travel between cities in every part of Greece, you can use the "KTEL" buses.

National Railways (http://www.ose.gr/)
For those preferring travelling by train... 

Ferries.gr (http://www.ferries.gr/)
Information mostly for travellers using ferries between Greece and Italy.