There's a tradition in the south of France on vocal improvisation. A guy has transposed it on the guitar, and he has tunes written out, but he usually divides his albums in 2/3 improvised tunes, and 1/3 jazz standards or written tunes... And he rocks!!!
His name is Sylvain LUC, check him out if you want to hear his stuff http://www.sylvainluc.com