Leafy greens like lettuce and spinach are cheap at the grocery store, but they're much better fresh. And if you keep them from bolting (putting forth flowers), you can pick them continuously for most of the year. Also, when growing your greens in the backyard, you don't have to worry about messy leaves decomposing at the bottom of your fridge because you didn't have time to eat them.
Another concern with store-bought greens is that they're susceptible to contamination. In 2006, for instance, an E. coli outbreak in packaged baby spinach caused 205 illnesses and three deaths [source: FDA]. Of course, washing greens is a good safeguard, but growing your own means total peace of mind. Lettuce and spinach grow especially well in backyards because they like a little bit of shade during the day and will wilt in intense heat.