Jessica of Running with scissors has a wonderful shark washcloth pattern (also a crocodile and a fish).
I put one together this morning, very easy. If you plan for adults to use this, make sure it's wide enough to accommodate their hands. The size I cut was as wide as an A4 sheet of paper, and I can't get my thumb in.
My seams were double-stitched with a zigzag, which helps keep in the bits and pieces of the cut terry cloth. It would be a great serger project. Also, if this is for a baby gift, make sure you use laundry soap that has no perfumes, etc.