Amaranthus viridis, for many Greeks the queen of boiled greens, the chorta they love with olive oil and lemon or vinegar. Some will even insist on drinking the water the vlita is boiled in. It grows like a weed in most gardens, and most garden shops in Greece will sell you the seeds. If you plant it once, though, it will stick around forever and grow exactly where you don't want it. Some Greeks say it makes you stupid but they all eat tons of it and still manage to get by.
Can also be spelled βλήτα.
Image by Joydeep, Creative Commons License