There are airbrushes that will do a very thin line, but none that will do a line as fine as a technical pen could, it is the nature of spraying the paint, so you will always get a feathered edge to it.
I might be speaking a bit out of turn, but there are few modelers/hobbist who actually freehand their fine lines. Most use a masking medium of some sort. I do a great deal of freehanding with my paint schemes, whether it be 'smoke rings' or those squiggly continues lines found on German aircraft. I saw somewhere that a modeler made friskets for them!
I was always in search of a 'better' airbrush to do thin lines, though I can get close, I still find the need in many applications to use masking for the finer ones which need consistancy in width and density of color.
So when you ask about a fine line airbrush, it depends on what your intentions and applications are needed to be known before a solid recommendation can be made. However, most double action airbrushes such as the Paasche VL with the smallest needle or the Iwatas do remarkable job at fine lines. It is as much of the operator as it is the airbrush.