I really like the line variety that dip pens give, but they are a little scary to take out sketching in public. I've been experimenting with a bunch of different pens trying to find something that gives nice line variety but can be thrown in a purse.
I'm really excited about the Pilot Parallel Pens. They come in different nib sizes and have replaceable ink cartridges that come in different colors.
They don't have waterproof ink though, at least I haven't been able to find any. This was black ink, but when I put watercolors on top it bled a beautiful purple color. I love the effect.
This sketch was done with a smaller Parallel pen, 1.5mm, and the black ink bled gray on this one. Not sure why they differ.