Sure, you might already know a few people who can spin a pencil around their thumbs. You might even be able to do it yourself, but here's something that will make you stand out in the thumb spinning crowd: spinning a pencil vertically around your middle finger! Here's how...

[edit] Steps

Starting position.

Hold the pencil the way you would when writing, except place it between the middle and ring finger, gripping it at the center. Thus, the eraser end of the pencil will be resting on the skin between index finger and thumb. It should be about half a centimeter from falling off of the skin. Stick the index finger out in a similar position, as when sticking the pinky out while drinking tea. The thumb should not be touching the pencil, and the middle finger should be on top with the ring finger below. The pencil should also be between the first knuckle from the fingertips of the middle and ring finger. The middle finger should be slightly more towards the eraser than the ring finger.
The ring finger and pinky should push the pencil up.

Let it spin.

Stiffen the ring finger and reinforce it with the pinky, which will be below. Push down with the middle finger, and dislodge the pencil from the skin between index finger and thumb. At the same time, push up with the ring finger and pinky. The pencil should swing smoothly around your middle finger.
End the trick in one of the following ways:
- While the pencil is nearly vertical, grip it with the index and middle fingers on one side and thumb on the other. The landing should be smooth and not be in the writing position.
- When the pencil spins all the way around the middle finger, it should land right in the skin between index finger and thumb. At the same time it lands, grip the pencil with any finger and any part of your thumb. The pencil should be able to land in the writing position or something similar to it. Either way is correct and acceptable.

[edit] Tips

Always make sure your fingers aren't too tight, or else the pencil will fly into the air, sometimes spinning wildly.
Practice this trick during your free time or while watching television.
Modify the strength used for the trick. The way described above may work for some but not for others.
After completing the spin, place the pencil back into the starting position with the other hand, if you find it difficult to do it with only the one hand.
To keep the pencil from making lots of noise when it falls, put a towel underneath it.
Place a tablecloth on the table, to keep the pencil from bouncing off the table and falling to the floor.
This trick is professionally referred to as the "Sonic."
As you improve with skill, move your middle finger less and less. Eventually there will be almost no noticeable motion except the pencil.
Once you have the hang of this, try starting the pencil in between your pinky and ring finger. Using the same steps, spin the pencil around your ring finger and catch it between your middle and ring fingers. You will now be set up to spin it around your middle finger - the double spin will give it a cool effect. You can even spin your pencil from the pinky/ring finger to the middle/pointer finger!
If you know how to perform a Charge, it can help to start the trick like a Charge, but bend down the middle finger and let it "slip through" into the next slot.
Learning how to perform this trick backwards (from index/middle slot to the middle/ring slot) is a good idea, because you can then perform this trick back-and forth continuously. It would then be called a "Sonic Harmonic".
Once you get the hang of it, try doing it without holding the pencil against the thumb flap. This helps for performing the "Harmonic" and linking this trick into combos, because it disrupts the flow if you must adjust the pencil against your thumb every time you need to perform this trick.
This trick can also be performed from the pinky/ring finger slot into the ring/middle finger slot.

[edit] Warnings

If the pencil flies into the air while spinning, it may hit someone around you as well as yourself. Do not use too much force when "snapping" the pencil. The "snap" should be effortless.
A retractable mechanical or unsharpened pencil will lessen the chance of injury.
If you decide to do this with a pen, be sure to keep the cap on. Ink from some pens tends to fling from the pen during the spin.