The nyckelharpa is a traditional Swedish instrument that has been played in one form or another for more than 600 years. At least four different versions of the nyckelharpa are still played today.
The modern chromatic nyckleharpa has 16 strings; 3 melody strings, one drone string, and 12 sympathetic vibration (or resonance) strings. It has about 37 wooden keys arranged to slide the strings. Each key has a tangent that reaches up and stops (frets) a string to make a particular note. The player uses a short bow with the right hand, and pushes on the keys with the left. It has a three octave range (from the same low "G" as a fiddle's 4th string) and sounds something like a fiddle, only with lots more resonance.
(all info on this page taken directly from the American Nyckelharpa Association web-site) Go to this site for more complete information http://www.nyckelharpa.org/
In Swedish, "nyckel" means key, and harpa, well, it's a bit harder to translate. I've heard older people refer to guitars and fiddles as "harpa". I think it used to be a generic name for stringed instrument. There are about 10,000 nyckelharpa players in Sweden today, due mostly to the tireless efforts of Eric Sahlström. The instrument almost died out at the beginning of the 1900's. Video below is 2 minutes long.
A popular Swedish folk group that features Olov Johansson on the nyckelharpa, Mikael Marin in the viola and Roger Tallroth guitar. (The folk dancers go by the name of LinnaeusDansarna and the dance is called Linnaeus Polonaise).
A film about the nyckelharpa from Uppland the birth place of the nyckelharpa.
The Nickelharpa Road Movie
The oldest "evidence" of nyckelharpa use is a relief (left picture) on one of the gates to Källunge church on Gotland from about 1350 depicting two nyckelharpa players. There are three surviving examples of the medieval nyckelharpa: one found in the town of Mora in Dalarna, Sweden (2rd picture); one found in Vefsen, Norway (3nd picture, it hangs in the Musik Museum in Stockholm, where the last picture was taken [i.e. that's a reproduction of the Moraharpa in the last picture]); and one found in Esse, Finland (4th picture). The Moraharpa is dated 1526, and hangs in the Zorn Museum in Mora. They have one row of keys (although some keys stop two strings, making them a mix of melody and drone strings) and two drone strings.
Note that the Moraharpa is shaped like a lute, while the Vefsenharpa and the Esseharpa are shaped more like the modern nyckelharpa. Recent evidence suggests that the Moraharpa was probably built in the early 1600's, using wood that was already about 100 years old. It was likely made as a copy of the picture in the German book (picture forthcoming). So it appears that the lute-shaped tradition is broken, while the long/narrow tradition is unbroken. A few people still play this version of the instrument today, including Anders Stake (now Anders Norrude) in the group "Hedningarna."