One of the major ancient sites of Greece some 178 km to the north-west of Athens.
Needless to say that this site is one of the most visited places of Greece.
Its spectacular location as well as its archeological importance is more than well worth the trip.
There is also an interesting museum.

Links to Delphi

http://www.greecetravel.com/delphi/
http://www.agn.gr/hellas/central/delphi.asp
http://www.arthistory.sbc.edu/sacredplaces/delphi.html
http://www.culture.gr/2/21/211/21110m/e211jm01.html