The Pentel Rolling Writer is an oldie, but goodie. It may have been a pen you saw kicking around in a junk drawer (likely dried out) or on your dad's desk, but now you have long forgotten about it...
Hands down, I think this is a great pen. For such a cheap pen, it is hard to believe it writes as "fun" as it does. It's almost difficult to describe the tip on the Rolling Writer. It is flexible and squishy (yet firm), but as you apply various amounts of pressure it changes your line widths. This makes for an extremely expressive penmanship and is, well, fun. I'd say it is pretty difficult to get a fine line with it, but I write with a pretty heavy hand. It is a fun pen to hand to someone to use and see their expression and they may just want to steal it.
They have a very classic look that hasn't changed in probably 40 years. The barrel is made of a cheap molded plastic and has a stamped metal clip. Nothing too fancy, but it still looks cool; vintage even.
They aren't terribly easy to find in your normal office supply store on the shelf, but you can order them online through Staples, although the only caveat is that you have to buy a whole dozen. But, the dozen is only $11.99 which is a total steal. If you work in an office and someone does the ordering for supplies, see if they can sneak in a pack of these next time...
(sorry the pictures are kind of dark on this one...)