I spent the weekend at this beautiful island (about 2 hours from Athens by hydrofoil). Spetses is mostly known for its great contribution (with warriors, money and ships) to the Greek war of Independence in 1821.
Private cars are totally forbidden so people use horse and carriages, motorbikes or these red sea-taxis to get around the island or to the nearby mainland shores. The little port's entrance gets very busy sometimes...
Spetses in Wikipedia
Spetses.gr (in Greek)
More info (in english)