Hakurei Turnips. I'll have to thin these, but they can be grown very close together. Small, sweet, tasty turnips. Very quick to mature--very like sweet radishes.
Some heading lettuces growing in window boxes. you can see how I use the spaces between rows here to squeeze in window boxes. When I need to weed or harvest, I just move the window boxes aside. You can grow a surprising amount of food in one of these cheap windowboxes--carrots, beets, turnips, lettuce. I've done it all. I use a mix of about 5::1::1 pine fines, peat, and perlite, with added micronutrients and high-end slow-release fertilizer. I supplement that with monthly feedings of liquid fertilizer. Essentially growing hydroponically in soil.
You can see these chard plants below, from the beginning of October, when I had just transplanted them. They're almost ready to harvest. Grown in a circle, cut from an old barrel, filled with very rich organic compost.
Onions and peas. I should have planted these peas much earlier than I did. They really appreciate a blast of heat at the start--I think I could plant them as early as mid-August.
Cutting salad greens. The two top producers for me are Bronze Arrow (Southern Exposure, I think...) and Apollo Rocket, a hybrid arugula that I love.