Today we're going to take a different tack and go with a more nurturing side of hacking: gardening. Dirt engineering is a down-to-earth activity that, IMHO, is good for the soul. It's surprising how many ways there are to garden; I had a plot in a community garden, and there were some divergent and dearly-held opinions about how to do things. I'll do a blog sometime about my favorite method, Square Foot Gardening.
Today, though, I want to throw out an idea I had: Weed gardening. I may not be the first to have this idea-- in fact, many have invented weed gardening spontaneously. It takes practically no effort at all! And that's the advantage. Now stick with me and hear me out. I'm not advocating agricultural anarchy-- there's more to it.
One thing everyone knows about weeds is that they grow better that the plants you actually want to grow. Our domesticated plants come from different biomes all over the world, and some are picky, whereas weeds, whether native or introduced, thrive everywhere. But there are two kinds of weeds. 
One kind is the noxious weeds, plants with nothing to offer that spread without limit, often poisoning other plants (really). The other kind is the sort that American Indians used to eat and make into cloth and use to treat fevers or whatever. In truth, every plant is a mix of these two, but we're going to focus on plants that are most like the latter.
By "weed gardening", I really mean the purposeful ordering, planting, and nurturing of improved varieties of plants that grow locally as weeds. For the first on our list, let's pick something very familiar. The French call this plant dent de lion, "lion's tooth", after the jagged appearance of the leaves. Before flowers appear, the tender, somewhat bitter leaves make excellent salad greens. After flowers, the leaves become more bitter, but would still be palatable as a potherb (a cooked vegetable like spinach). 
The flowers themselves are edible, substantial, and mildly flavored, and can be made into wine or fritters (dipped in batter and fried). As with lettuce or any green, letting the plant go to seed should be avoided by picking off blooms that have gone too far or harvesting the entire plant if blooms become overwhelming. The roots can be roasted for a chicory-like coffee substitute, and many swear the plant has medicinal properties.
If you haven't guessed by now, this is the dandelion. Improved varieties are available-- plant these, not actual weeds.
More about growing dandelions can be found here. Some of my favorite seed providers are Pinetree Garden Seeds (cheap and a good variety) and Johnny's Seeds (more expensive, but an amazing array of specially bred plants). A good source of unusual and native seeds are available from Redwood City Seed Company. Improved dandelion seeds can be found here and here.
I live in the Rocky Mountains, so I know what weeds grow here, and they are the ones I'm going to talk about. A chat with your local nursery will probably teach you volumes about both weeds and domestic plants.
Next up: amaranth. Amaranth seed was considered a wonder food by the Aztecs, and is highly nutritious and palatable. My wife loves it. The leaves can also be eaten (go easy on them at first-- young ones raw, older ones cooked). Of course, the plant grows like a weed! It enjoys lousy soils with little water. But the best thing about amaranth is the sheer variety. Amaranth has been bred into several showy ornamental varieties that also produce edible seed; in fact, all the photos in this blog are of different amaranth varieties. (From top to bottom: Red-Green Calloo, 'Hot Biscuits', Opopeo, Globe var. 'Mardi Gras', Asia Red, 'Golden Giant', Asia White, Love-Lies-Bleeding, and Joseph's Coat.) I've considered a garden made of nothing but amaranth. The Seedman has several varieties, and looks to be a good source for other unusual seeds. The Domestic Goddess also has a list of varieties here. You are sure to find much more on the web.
Quinoa is a related wonder grain that is cold-tolerant, and Celosia is a pretty related genus (although I don't know about its edibility).
I could go on and on, but I just don't have the time. So the rest of the weeds won't get much more than a mention...
Purslane is a sprawling succulent that tastes something like prickly pear, but not as sour. Mexicans and the French like it. Seed is edible. Rose moss (portulaca) is an attractive and hardy relative, although I don't know how edible it is. Purslane is very hardy; if a careless weeder throws the plant on dirt, it will reroot.
Salsify or oysterplant looks like a small fountain of narrow leaves aboveground. It produces one large flower that turns into a giant dandelion puff. But the real treat is the long taproot that has a unique flavor some describe as oysterlike (but not very). 'Sandwich Island Mammoth' is a common improved variety.
Mallows are a sprawling herb with round, hairy, bumpy-margined leaves. The immature seedpods resemble tiny wheels of cheese (although segmented). The leaves, flowers, and seedpods are edible and somewhat mucilaginous. Improved varieties include curly leaf, red chinese, zabrina/zebra, and "miniature hollyhocks". Related to hollyhocks.
Buckwheat is related to rhubarb and produces a seed that is popular in pancakes. There are dozens of species called buckwheat, and frankly, I can't make heads or tails of the whole mess. But just plain buckwheat is easy to identify.
Tiger lilies have become naturalized here-- besides being pretty, the unopened flower pods can be incorporated into Chinese cooking. They taste a little like an oniony green bean.
Other weedy edibles and medicinals include cattails (a favorite of survivalists, but need marshy soil), mint (invasive!), plantain (not the bananas), nasturtiums (love lousy soil and taste like watercress), Mexican tea, prickly pear (a very thirsty spineless variety is available in California), and maybe even oyster mushrooms (not a plant, but the easiest mushroom to cultivate).
Pretty weeds include milkweed/butterflyweed, blue flax (can be made into cloth, but invasive), jimsonweed (pretty but poison), prickly poppy (pretty and prickly and poison), and cleome or spider flower. 
Ersatz weeds: Deadly nightshade is also pretty, but it is perhaps better to substitute garden huckleberry, a related plant that produces an edible berry (if you harvest it just right). Nobody in their right mind would plant field bindweed (usually called morning glory), but genuine tropical morning glory is eager to grow-- and dies each winter. (Just beware of reseeding.)
Ask a local nursery or agricultural extension service for help with your ideas. They're sure to love it. After all, when you think about it, every plant was once a weed!
**** UPDATE*******
NPR's Science Friday for May 22, 2009 had a segment on revitalizing wild ecosystems by replacing grass with native local plants. Check it out.
I cannot believe I left out sunflowers! Sunflowers love crappy soil, hot sun, and dry weather. They look terrific all season long, produce edible petals and seeds, and they come in zillions of beautiful varieties! Check out 'Infrared', 'Teddy Bear', 'Lemon Ice', and 'Sunspot'.
Chicory is an unusual plant with pretty blue flowers. The young flowers can be eaten raw or cooked. The roots can be roasted for "coffee" (that's Postum, folks). And if you bury the living root in sand overwinter, you'll get Belgian endive. Whoa.
