As a complement to SSSB SSCO has started up a subleasing agency called Akademisk Kvart. Here you can rent a room or an apartment in second hand and it´s easier to find somewhere to live faster. Although, since it´s subleasing it might not be as secure as a first hand contract, so make sure to get a signed deal with your landlord.
At jagvillhabostad.nu you can find some good advices when looking for housing in Stockholm. See especially their Take charge – Your survival kit in the housing jungle manual.
Ledigalagenheter.com leases appartment from private landlords i.e. appartments that you won´t find in the housing queue of Stockholm City.