Today I bought a few Tombow Dual Brush Pens in a couple of shades of gray and one brown -- I already have a dark brown one and love it. They have a brush at one end and a point at the other, and the ink is water-soluble. They're great for quick sketches that you can add a little shading or pizazz to by adding water.
I'm planning to add these to my "art to go" sketch kit bag. In the meantime, I tried one of the gray ones out on a sleepy Truman at home this afternoon: