OTE, the ponderous but no longer sadistic Greek telephone company,  maintains but does not advertise its reasonably powerful English-language on-line telephone directory.  The on-line White Pages are also useful for looking up a name (and sometimes address) from caller ID on your telephone. Be warned, of course, that many telephones are still listed in the name of someone's uncle who died 15 years ago.  Transliteration of Greek to English is a dubious art.  Therefore, truncate names to the first few letters when you can, to minimize chances for mistakes.  Use only CAPITAL LETTERS.  Athens is ATHINA	http://whitepages.oteshop.gr/en/index.jsp
The Greek Yellow Pages (Chrysos Odigos) is incomplete and uses an idiosyncratic breakdown of business categories, but is nevertheless better than wandering aimlessly.  You can find a business in your neighborhood by asking it to search in the first three or four digits of your postal code.	http://www.xo.gr/index.jsp
Most internet portals on Greece rapidly run out of steam.  gogreece seems as good as any as a basic web directory in English...	http://www.gogreece.com/