The same plant as beets, actually, but with cultivars bred for the production of tasty leaves. Chard stands for quite some time in the heat without going to seed, and it is cold-tolerant as well. As an added bonus, some varieties are positively ornamental with their brightly-colored stems.
Soak seeds for 24 hours before sowing. Plant 3 inches apart in rows, thin to 6 inches apart (and eat the thinnings!).