I was introduced to ruling pens by Stephen Gardner. They're a lot of fun, because they tend to fight with you, which makes for a more energetic sketch.
Look at me talking about ruling pens like they're new. These things are older than I am in origin.
Area 221's Sketch-a-Day-in-May Celebration - http://area221.blogspot.com/
Stephen Gardner - http://www.gardnerillustration.com/