I can't believe I neglected to add the link to this site's sidepanel yet. Well, now it's there. There's such an insane amount of good sketching going on "over there". Actually it's all around. I'm not going to repost it all. Just check it out: Urban Sketchers.
The piece below is a great example from lapin barcelona. I've been trying to figure those brush pens out myself.
Also try this one out:
Above also by lapin barcelona.