I just came across an interesting cartridge refill pen when I was searching for gum arabic in an art store.
It takes a small international ink cartridge but instead of having a solid metal nib, it has a roller ball.
It's made by a German company called Herlitz and description is "Roller ofr ink cartridges, handy pocket size".
It's made of plastic and is about the size of a Pilot Petit1 and costs about US$5.
The standard size cost less about US$3.
It can be found here: www.herlitz.de, writing instruments, cartridge pens.
Unfortunately the picture doesn't download.
Out of curiosity, I bought one to try. It's very smooth and the ink flows at the lightest touch like a traditional fountain pen. whilst doing some scribbles, occassionally it emits very faint squeaks and has a little vibration. probably due to the rotating ball. The written words exhibited subtle shadings like when writing with a fountain pen. It doesn't need to be held in a certain way to form words and can write in any angle.
Under strong light, I could see the fins in the translucent section.
One of the best thing about it is that it can be left uncapped for prolonged periods (thinking time) and the point doesn't dry out! Starts immediately.
Would u consider this a "fountain pen"?