Hostels
Hostels are the obvious budget option if you're happy to sleep in a dorm. However if you want more privacy many hostels now have single and twin rooms, some are even ensuite but the price is higher. Be aware when comparing pirices that some hostels charge extra for linen, towels, blankets and breafkast.
Hostelbookers.com have no booking fees and guarantee you the lowest cost hostel booking.
Hostelworld.com is also worth looking at. I found cheap accommodation in single rooms in student residence flats over the Summer in Aberdeen and Edinburgh in Scotland.