I carry this one in my tobacco tin kit:
http://www.bensbackwoods.com/servlet/Detail?no=129

"Eze Lap keychain sharpener w/fishhook groove
SKU: EZLH
NEW EZE LAP KEYCHAIN DIAMOND SHARPENER IN 600 GRIT,ALSO HAS A GROOVE TO SHARPEN FISH HOOKS AND SUCH.VERY HANDY ON A KEYCHAIN...COMES IN DIFFERENT COLORS (random pick, all are 600 grit) MADE IN USA
PRICE: $5.99"

It is very small, flat and thin, maybe 3/4 x 2 x 1/8 inch.

For everyday home use, I use the Lansky Deluxe Diamond Knife Sharpening System or the Spyderco Tri-angle Sharpmaker. In the field, I use one of those ones with the foldover handle and diamond-embedded little disks on about a 1 x 4 inch plate. I am well satisied with all of them.

Jeff

Hi,

I also carry the "Eze Lap Keychain Sharpener" that Jeff mentioned above. I bought it to put in a small survival/hunting kit but it is so handy (with key clip removed) that I have started carrying it in my wallet. It does a reasonable job of touching up a small knife in the field but you have to be very careful as it is easy to cut the fingers of your holding hand (I have a bandage on my thumb right now).

One trouble with having a sharpener on you is that if someone sees you using it you end up sharpening everyone elses knives, not an easy task with a sharpener this small.

Mike