In Norwegian folk music, the most used instrument is the hallingdal fiddle. Most Norwegian folk music, such as Stevs and Joiks are ususally acapella-only, which means no instruments are used. Stevs used to be popular to perform until the late 1990s. Some popular kveders (people who perform Stevs) from the 70s, 80s and 90s are Arve Moen Bergset, Kirsten Bråten Berg and Øyonn Groven Myhren (The latter is my absolue favorite Kveder)
In Swedish folk music, the Nyckelharpa (Key harp) is used a lot. The Jew's harp, which despite the name is used a lot in Nordic and Baltic folk music is also commonly used