To take advantage, check out the Norwegian sale page, which lists out the cities they fly out of. Once you've clicked through, they list a lot of one-way prices, showing the best prices available to the destination listed. It's an intuitive system, even if it fails to give you an accurate sense of what roundtrip tickets will cost.
Here's a taste of what you'll find. Flying out of New York City you can get to Stockholm for $283, Oslo for $335.10, and Copenhagen for $394.40. From Boston fly to Oslo for $284.70, Copenhagen for $338.40, and London for $406.40. Fort Lauderdale has flights to Stockholm for $317.50, Oslo for $342.20, Copenhagen for $410.40, Barcelona for $414, and Paris for $424.60.