PITT Artist Pen by Faber-Castell
I've been using this PITT artist pen a lot lately. The ink is waterproof, lightfast, acid-free, and pH-neutral. It comes in four nib sizes: superfine (S), fine (F), medium (M), and brush (B), which is prominently displayed on the end of the cap.
The ink seems to go on bit smoother than my Pigma Microns. I've also been using the brush pens on some illustration work I've been doing with good success. If you haven't tried these pens, give one a try for your sketching work!