The nyckelharpa is a traditional Swedish instrument that has been played, in one form or another as it evolved, for more than 600 years. At least four different versions of the nyckelharpa are
still played today, an uncommon situation for most folk instruments.
The modern chromatic nyckelharpa 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 under 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 3 octave range (from the same low "G" as a fiddle's 4th string) and sounds something like a fiddle, only with lots more resonance. Earlier forms of the nyckelharpa had fewer keys, fewer (or no) sympathetic strings, and fewer melody strings, but often made more use of drone strings.
In Swedish, "nyckel" means key, and "harpa", well, it's a bit harder to translate. I've heard older people (my grandmother's generation) refer to guitars and fiddles as "harpa". I think it used to be a generic word for "stringed instrument" (which makes sense if you think of the most generic or basic instrument -- some open strings on a box, a harp). The nyckelharpa has historically been called "nyckelgiga", "nyckelspel" and "nyckel-lira" as well. "Giga" and "lira" are both generic words referring to instruments (c.f. lira -> lyre like harpa -> harp) and "spel" means "play". Sometimes people call it the Swedish key-fiddle, but nyckelharpa isn't that hard to pronounce (see the first sound link below).
There are about 10,000 nyckelharpa players in Sweden today, due mostly to the tireless efforts of Eric Sahlström. The instrument almost died out during the first part of the 1900's, but by riding the 'green wave' of the 1960's and 70's it has made quite a come-back. In the book "Uppländske Spelmän under 400 århundraden", a history of folk music in the province of Uppland during 1550-1950, there are 249 nyckelharpa players listed. Per-Ulf Allmo is currently working on a book about the nyckelharpa, and has found another 700 pre-1960 players, bringing the total of 'famous' (enough to be remembered) historical nyckelharpa players to 949.