In his 'Penguin Atlas of Medieval History' Colin McEvedy states that the last pagans were around at the time of Heraclius (early 7th century). I don't know what his source was, however.
A year or two ago on BBC I saw a news article about modern greeks who have started worshipping the old gods, dressing in ancient greek clothes and going to Delphi etc etc. As an agnostic verging on atheism I say good look to them, at least it is a religion directly ancestral to their own culture. The thing that made me laugh was an Orthodox bishop, saying they were infantile and that their religion was nonsense. Really? From someone who believes that a guy walked on water and rose from the dead?