In every 7-Eleven store I found in downtown Stockholm there was a wireless network provided by Rover Rabbit. These had a very strong signal outside the store too (I believe that the dipole antenna over the doorway is in fact the Wi-Fi antenna, which would explain the signal quality!).
They are a pre-paid only provider, selling access in units of 1 hour, 24 hours or one month. Unlike most pre-paid services though they expire on calendar time after first use, irrespective of the amount of use.
These hotspots are now supported through Devicescape now – just enter the username & password from the scratch card, or, if you buy online, activate with your browser and use the random looking username/password from the popup window (your Rover Rabbit website username & password will not let you get online sadly).