Subway:
Porto's relatively new subway system has frequent departures to the city. Most destinations shouldn't take more than half an hour to reach.
The line reaching the airport is the Violeta line (E).
Operating hours: 6:00-1:00
Ticket: 1,35€

Buses:
An Aerobus runs to the main hotels and the centre of Porto.
Ticket: 4€
There are also regular buses which serve the city's centre (601, 87).
Ticket: 1,20€ 

Taxis:
Taxis are available 24h. A taxi ride to the city should cost around 25€ (The price indicated by the taximeter plus surcharges for baggage or other exceptions).