I stayed at the Generator, and paid 9 pounds a night, which was definitely decent for London!
The pros: free breakfast, clean, right by a tube stop that goes right into the sightseeing areas, lots of people, bar downstairs.
The cons: lots of big groups, and so many people making it not a typical hostel feel, but a bit more institutional, I guess, loud and stuff too
I also stayed at Dover Court or Dover Place or something like that. It was pretty dirty and not a great location, but there was opportunity to meet people. I would stay at the Generator over Dover though.