Sharpie has combined the permanence of a pen with the erasability of a pencil with their new Liquid Pencil. The "ink" inside this pen is a liquid graphite that stays erasable for three days. After that, it turns to ink and stays permanent.
It's a pretty neat trick! The pens go on sale starting next month for not much more than the price of a normal pen. The only downside is that, well, I never really use pens anymore. If only they had figured this out a decade or two ago!