The Piraeus is the ancient port of Athens and still functions as the chief exit point from the city by sea for destinations amongst the Aegean Islands and elsewhere in the east Mediterranean. Domestic destinantions include all of the Aegean islands except the Sporades and some smaller Cyclades and Dodecanese isles that require a connection. International destinations (apart from cruise ships) include Cyprus and the Middle East.
Warning: Recently, official spelling changed to Pireas -- and all road signs in Athens and around updated accordingly.
Piraeus occupies is really A huge territory. Check your gate number  in advance so you know where to go (it may take 15 minutes of driving / 30 minutes of walking to reach one end of the port from the other).
Since privatisation of the Port of Piraeus Authority  passenger facilities have improved greatly. Air conditioned tents have been set up at departure locations around the port and free wi-fi internet access is now available.
Sailings (arrivals and departures) are posted outside the Coast Guard Building at Karaiskaki Square, the main bus terminal for the Piraeus suburbs. Sailings are also available online  from the Ministry of Merchant Marine .
The area can become hectic, especially during the summer period, so allow plenty of time to navigate to the correct departure point.
Most travellers arriving in Piraeus from Athens make use of the very convenient Metro . Line 1 terminates at the Port, from there it's a short walk to the Saronic Gulf ferries, hydrofoils and catamarans, or a free shuttle-bus ride to the ships sailing to Crete and the Dodecanses. Central Cyclades ferries conveniently sail from just across the metro station. Metro tickets cost €0.70 for short journeys on Line 1, or €0.80 for longer trips on the line, or connections to Lines 2 and 3. A €1.00 ticket allows unlimited connections in one direction on all modes of transport.
Direct Airport Express buses  run 24 hours between the port of Piraeus and Athens International Airport . Allow 90 minutes for the trip. The Airport is also accessible via the Metro, with a connection at Monastiraki. Bus tickets to the airport, available from the driver, cost €3.20; Metro tickets to the airport are €6.00.
Other public buses connect Piraeus with its outlying suburbs, the souther coastal zone and with central Athens. Bus and trolley-bus tickets for single journeys cost €0.50. They must be bought in advance (generally from kiosks) and validated once on board.
The centre of Piraeus and the Port can be negoatiated easily on foot if you are not carrying luggage. Yellow trolley buses are useful for the run from the Port to Passalimani and then follow the very scenic route around the hill of Castella, terminating at the Line 1 Metro station of Neon Phaleron (Neo Faliro), near the Peace & Friendship and Karaiskaki Stadiums.
Free shuttle buses inside the Port run from across the Metro Line 1 Terminal Station, around the north side of the port to the ships sailing for Crete, the Eastern Aegean and the Dodecanese.
The most important thing to do is to take a ferry to the Greek Islands. You also can have a nice walk in the harbour and look at many amazing yachts and ships.
There isn't much to buy in Pireaus. Wait until you get to your destination to begin purchasing souvenirs.
Delfino Restaurant . Located in Mikrolimano, a cosy bay with hundreds of sailing boats. Excellent seafood restaurant - try fresh fish on the grill and fresh lobster with linguini. Warm and profesional service. Amazing the indoor and outdoor areas. For mid/high budgets. 60, Akti Koumoundourou, Mikrolimano, Piraeus, tel. +302104120388.
Good cafes are very scarce in the port.