Although Apple refuses to make a touchscreen Mac, the Espresso Display V2 is ready to fill in the gap. There are 15.6-inch and 13.3-inch versions of the very slim external display, and both support touch and an optional stylus.
I tested the screen with a MacBook and iPad to see how well it performs in ordinary use. And I also tried out the stylus as well as the stand for desktop use and the case for taking the display on the go.