This Polska från Hälleforsnäs has in the last 10 or 15 years become one of the most popular Swedish fiddle tunes worldwide.
It is an old traditional slängpolska from Södermanland, but found its place in nyckelharpa workshops taught in many countries outside of Sweden. It is now played by nyckelharpa and fiddle players, on guitar and flute, recorder, hurdy-gurdy, mandolin, and even banjo!
Here is a YouTube video of Magnus Holmström, a prominent Swedish nyckelharpa player and teacher, demonstrating the Polska från Hälleforsnäs. The video includes a slow version where he shows the double-stops he is using.
And here to the left is a video of me playing it on the fiddle.