Walnut (Juglans californica) comes from the west coast of the U.S. It has a rich pleasing aroma and warm dark brown color. Well-quartered backs and sides are readily available as well as a nice selection of curly, flamed, and burly figured pieces (at higher prices, of course). Walnut is considerably lighter than rosewood, but comparable to koa or mahogany.
Walnut imparts a bright woody sound to a guitar, though with more warmth than mahogany. Played at low levels, walnut is bright and clear with strong fundamentals. At higher volumes, though, walnut's overtones kick in and it has the punchy sound of rosewood.