'Man is the measure of all things.'
- Plato
Ol' Cave-boy was a bit more right than he knew. The human body is used as a measuring stick for all sorts of things.
The most obvious part is 12 inches, or, the foot. Fair enough. In French, a foot is still a pied, but an inch is a pouce, or a thumb's width. So all the thumbs of Frenchmen are one inch wide. But don't ask me about those Kay-bekkers, all right?
A span is the distance between the tip of the thumb and the tip of the little finger on an outstretched hand. By convention, this is 9 inches. A hand is four inches (you'll see that measurement on horses). An ell is 45 inches long (a cloth measurement, IIRC) and a cubit is anywhere from 17 to 26 inches long, but both are based on the distance from the elbow to the tip of the fingers. Ell for el-bow, and the Latin cubitum means elbow. A fathom is the distance between the tips of your fingers if you stretch your arms real wide (six feet, by convention).
Then you've got a few folksy-type measurements. Three fingers is a stiff drink (the height of three fingers sideways in a shotglass). Supposedly, in the 50s, ten fingers meant about five feet - the height of an object that a friend would have to boost you up onto (using his two hands, or ten fingers). Your limbs are no longer legal tender, but expensive things still cost an arm and a leg. A heartbeat is a near-instantaneous length of time, about equal to a twinkling, which is but a microwink, forty winks being a good nap.
There was once a man named Smoot... no, this isn't a dirty limerick. Oliver Reed Smoot was an MIT student who offered up his body as a way to measure the length of the Harvard Bridge. After much tedious getting up and laying down, the length ended up being 364.4 Smoots and one ear. So now, MIT students crossing the bridge know that they have exactly 186 Smoots to go until they cross the bridge.
And, my favorite one : stretch your arm out, turn your hand perpendicular to your line of sight, and keep your fingers together. The height of four fingers held sideways at this length is almost always 8 degrees, no matter what. This is because finger thickness tends to be closely related to arm length, so the thicker the fingers, the further they'll be away from your eyes, etc.
Now, you may be pro-metric and all, but admit it : these units are much more fun than the oh-so-dry meter, which is now officially [1/299,792,458 the distance traveled by light in a vacuum in one second.