If you live in the beautiful desert Southwest as I do (Arizona), You've probably seen Prickly Pear Cactus. The name is quite descriptive - They are prickly (how prickly depends on the variety; some only have hair-like thorns.), and the "Pads" are vaguely pear-shaped.
Cacti are tremendously tough and hardy plants. About the only way to kill one is either poison it or give it too much water! (Note for you Eastern transplants: Do Not Water the Cactus!) They are also ridiculously easy to plant where you want them, so this will be a very short Instructable.
More info on Prickly Pear:
Step 1: Find a Prickly Pear Cactus
There are several varieties of Prickly Pear; the one shown here has long wicked thorns that are great for keeping prowlers away from your windows or making a natural fence. Some grow upward, and some stay low to the ground and spread. One variety has elongated pads, some have attractive purple colored pads. They are not protected by law as Saguaros are, but don't take them from a National Park or Monument. There is plenty of desert outside of those areas to look in. Find a variety you like.
Step 2: Knock Off Some Pads
Using a broomstick or other tool to give you some distance from the thorns, gently break off some pads. You probably don't have to hit the cactus, just push firmly and the pads will break off at the joint. The less damage, the better chance they'll have. They propagate naturally this way; When the pads get too heavy (With water, which means conditions are good for propagating), they fall off and sprout.
Step 3: Throw Them on the Ground
Just toss several pads on the ground where you want to grow Prickly Pear. It's not necessary to water them, although if it's the dry season, you might give them a little water (as in 1/4 cup! they don't need much encouragement!) for a week or two. I generally just toss them down and forget about them, and about 60% of them sprout.
Step 4: Ignore Them!
Cacti, more than any other plant, thrive on neglect. Even if you forget about them, even if you give them no care or water whatsoever, some of them will sprout and grow.
For the first few weeks, they'll look like they're dying. They'll curl up and look sick. That's because they're putting all their stored water and food into making roots. Some won't make it, but wait until they're completely dried out and looking like potato chips before you give up on them!
Desert landscaping is fun! Pretty soon, you'll be trimming the cactus because there's too much of it...