I've always wanted to learn to play the guitar, but all those strings are confusing! So perhaps this is the easier instrument: the Longbow "American Classic" 2 string bass!
http://www.longbowbass.com/Gallery/tabid/55/Default.aspx - via Guitar Blog (who just celebrated its 6th Blogiversary. Sixth! Congrats!), thanks Stratoblogster!
(There is, of course, a one-string bass guitar from Japanese guitar maker Atlansia, the Dàn bâu from Vietnam and the Duxianqin from China)