For low useage, from Telepriser the cheapest option is ludo, as they give you 185kr per month to use as you please, this corresponds to just under 50mb a month for free, after that it is 4kr/mb. My next recommendation for would be onecall, who charge 1kr/mb but have datapackages for data, 500mb for 169kr.
Although both of these require that you have an address to receive the sim card. Otherwise i think you are only left with the ones that you can buy in the shops, generally telenor, netcom and lebara. Hope this helps.