Guest, House is quite right, a lot of the examples quoted here are of drone accompaniments to songs which would conventionally use several chords. For instance, "Over the Hills and Far Away" would normally be a three-chorder, but John Tams plays a very effective fiddle drone to it in "Sharpe" (I know, 'cos I bought the boxed set for Christmas). If you want real one-chorders, look at old blues, as has been suggested, or old-time banjo stuff. Dock Boggs "Country Blues", if memory serves, has just the one mdoal chord throughout. Some Boozoo Chavis zydeco as well.