I can recommend
www.hostelworld.com/
You can finetune your search and only look for budget hotels and appartments and you can search on minimum rating and price as well. They do campsites, hostels and budget hotels all over the world. I've stayed in a number of budget hotels that I booked through hostelworld and often ended up in really nice places and sometimes really interesting places. Only once I stayed in a place that I didn't like very much.