By far and away the best deal is a roundtrip from San Francisco to Stockholm for only $399.
There is a large number of fares that will get you to Europe for well below $500 including New York – Madrid, Seattle – Frankfurt, Denver – Paris, Boston – London, etc. etc. It seems that each USA gateway city has at least once outstanding deal.
Departures must take place between February 2 and April 6, 2017 and return travel must take place between February 2 and October 6, 2017. Tickets must be purchased at least 7 days in advance of departure and surcharges may apply for weekend travel (Thursday – Sunday flights).
The fare sale lasts through January 31, 2017.