Anavasi Maps prints the best hiking maps, very detailed topographic
maps with GPS coordinates. Their maps are available at
Eleftherodakis and other major bookstores.
Road Editions was the first company to produce real, accurate maps
of Greece. Their first love is motorcycle touring, and their
guidebooks (mostly in Greek but a couple in English) are superb for
getting off the beaten track and finding beautiful scenery and a
good meal.
A detailed map of Athens, including a searchable street directory,
is available on-line. However, execution is painfully crude,
and street names are only in Greek.