If you are looking for housing in Seoul and you are on a budget or if you are a student then check the following website:
This link has a very good list of guest houses starting from 10USD/night, so click here .
I will add up to this myself but if you know any useful website either in English or in Korean then please do share.
My post on Hasukjib accommodation is here.
My post on various types of accommodation options for students in particular and short term travelers to Seoul is here.