GH or hotel in Porto

You could try the lowcostholidays (Hotel Only option) or the hotelscombined websites.

Last year i've enjoyed staying in Residencial Pão de Açucar. At the time i've also considered Grande Hotel do Porto which was around the corner. Both include breakfast in their rates.

Check out booking.com and hrs.com where both hotels and many other places are offered and reviewed.
Porto is relatively cheap on accommodation.

CONFORTABLES HOTELS IN CENTER CITY :
- castelo santa catarina(charme): http://www.castelosantacatarina.com.pt/ (pk free)
- grand hotel do porto (charme):http://www.hotelclub.fr/Grande-Hotel-Do-Porto/
- quality inn -praça batalha:http://www.qualityinn.com/...porto-portugal-PT025
- hotel america : http://www.hotel-america.net/ (pk free)

OTHERS HOTELS SUITABLES AND NOT EXPENSIVE IN CENTER CITY:
- residencial rex:(pk free):http://www.activehotels.com/...ncial-rex-porto.html
- residencial pao de acucar:www.residencialpaodeacucar.com
- seculo residencial:http://www.seculoresidencial.com/
- hotel eurostars das artes:http://www.eurostarshotels.com/EN/hotels-oporto.html
- residencial paris:27 rua da fabrica:tel222073140
- pensao mira d'aire:197 rua alvares cabral.tel.222083113
- pensao estoril:http://www.pensaoestoril.com/location.asp?Lang=2