I frequently make ink drawings, embracing anything from hand-rubbed liquid from Chinese ink sticks to ball pens, both water-soluble as well as non-water-soluble. Initially in college I considered them as ideas for large scale paintings, under the impression that all important art has to happen on a large scale. Eventually I put more value onto them, recognizing and creating them as their own individual pieces.
Ball-pen is still a favorite medium of mine, which I prefer over more expensive refillable pens. I am keeping an ongoing collection of various black ball-pens from all over the world, unfortunately once they are used up it can be hard to replace them if they come from far away or if they are no longer being produced.
The Usagi-chan work (see separate category) is mostly made with ball-pen.