The Greek National Tourist Organisation (www.visitgreece.gr) is known as GNTO abroad and EOT within Greece. The quality of service from office to office varies dramatically; in some you'll get information aplenty and in others you'll be hard-pressed to find anyone behind the desk. EOT offices can be found in major tourist locations, though they are increasingly being supplemented or even replaced by local municipality tourist offices (such as in the Peloponnese).
The tourist police also fulfil the same functions as the EOT and municipal tourist offices, dispensing maps and brochures, and giving information on transport. If you're really stuck, the tourist police may be able to help find accommodation.