A gel roller ball pen refill made by Parker. The cool thing is that it fits into any ball point pen that takes a Parker-style refill—even vintage pens. For a small cost, a wide range of pens can be upgraded to a roller ball. A variety of pens can take this cartridge. So far, I have found that it works in a Parker Jotter, 75, and a few cheesey marketing pens with the company logo on their side. Further, though I haven't tried it, removing the plastic thing on the end might allow it to work with a Fisher Space Pen.