Private hostels proliferate in Kraków and Warsaw and have cropped up in other cities as well. They generally offer excellent service, with internet access and laundry, for around 45zł per bed, but during the Polish summer holidays (July & Aug) it is advisable to book ahead. Even the smallest towns have a public hostel (schroniska młodzieźowe), with dorm beds for around 30zł; for a complete list check w http://www.ptsm.org.pl. In large cities they’re centrally located and open year-round, though often with lockouts and curfews. There is at least one budget hotel in every town, with 120zł normally enough to get you a Spartan but habitable room with communal toilet and shower. Many tourist offices can also find you cheap rooms in private houses (kwatera prywatna; 70–80zł).
Polish campsites are often a fair distance from town centres and are not always much cheaper than a hostel dorm bed (20–40zł). Though some of these sites have excellent facilities, in others you’ll find a toilet and little else. For a list of campsites in Poland, check w http://www.eurocampings.co.uk/en/europe/poland.