When it comes to tools for integrating freehand art into the digital workflow, Wacom remains the leader. Its tools aren't perfect--and certainly aren't cheap--but it's hard to find any more respected. And when it comes to drawing, nothing yet beats pen (or pencil) and paper for fluidity and portability. Wacom attempts to combine the best of both worlds with its Inkling, a pen and receiver combo that tracks your penstrokes for up to 50 sketches and uploads them to a computer for use with applications like Photoshop or Illustrator.
This tutorial video from Wacom shows you how it works:
Basically, you attach the receiver to the paper, sketchbook, whatever, then draw. You can create new layers at the press of a button. When you're ready, you connect the receiver to a computer via USB and upload the files to Wacom's Sketch Manager software, which transcodes them to whichever file format you need. The system is pressure sensitive, with 1,024 levels of sensitivity--as good as Wacom's last-generation pro input tablets.
It sounds great, but there are also some obvious drawbacks. First, it's $199. That's fairly expensive for a lot of nonprofessionals and students. It's limited to A4-size paper (a little larger than letter-size, for U.S. denizens)--I'm not really sure if that's a problem or not.
It also requires line-of-sight between the pen and the receiver, and you can't draw within 0.8 inch of the receiver; in such a confined area I'm not sure that will be as much of a problem as some people have been making it out to be, though if you hold your pen oddly you might have some issues.
Finally, the nibs are standard ballpoints, and according to Wacom, there are no plans for gel, felt-tip, graphite, or other sorts of nibs, so if ballpoint isn't your thing, you're out of luck. And while you can delete layers in software, you can't erase mistakes.
But warts and all, Inkling makes me wish I could draw better than your average 5-year-old. It's slated to ship in September.