I've been doing this ever since I've had a patch of dirt in which to grow 'em. Next time you grab a bunch of scallions/green onions at the supermarket, hold onto the root ends.
Then, simply plant them about one inch into the dirt (in the garden or a pot). Water well, and wait for the greens to return. The more you snip them, they faster they'll grow.
You can also do this with whole garlic bulbs and use the green sprouts in pastas, stirfrys, etc.