Well, there's the old standby of a violin bow. Jimmy Page used to do that, and I have a number of both Kontakt and Omnisphere samples that do that. Not all of them sound good, though. Although Omnisphere's bowed piano is kinda cool.
I have also seen spoons used to hit guitar strings, which produces a bouncy sound similar to that of a hammered dulcimer. I think they used that method on a few Omnisphere samples, too. Must be really hard to hit just one string with a spoon, though.