Buckwheat
One of the most under-used crops. Buckwheat is a gluten-free grain alternative, yes. But it's also one of the only crops that can pull phosphorus out of mineral soil and make it available to plants. It's one of the best nectar plants for beneficial insects of all kinds. And it's an incredibly fast-growing, weed-smothering cover crop for summer.
If you don't think you need a cover crop for summer, stay with me. Do you really plant every square inch of your garden right away in spring? If not, consider sowing some of this at your last frost date and let it build fertility, prevent weeds, and draw beneficial insects until you want the space. It's the perfect bridge crop for after your overwintered or early-spring crops bolt and before planting fall crops. Do you have bare spots in summer as crops finish up? Buckwheat only needs 40 days to make a stand of lush greenery and nectar-rich flowers. And if you find that you're over-extended, have travel plans, or have weedy problem areas, plant buckwheat to keep weeds at bay and build fertility while you're dealing with other stuff.
One little-known but really effective technique is to use it in spring for a nurse crop, especially if there is wind or pest pressure. You sow the bed with buckwheat, then hoe out little openings in the buckwheat for your transplants--cabbage, squash, or whatever. The buckwheat protects the little plants and distracts the pests until your transplants are big enough to own the space. You can cut down the buckwheat at that point and either compost it or use it for a nutrient-rich "chop and drop" mulch. Or feed it to chickens. Since it's not frost-hardy, it can also be planted in August, then winter-kill and act as a nutritious mulch until spring. Oh, and if you end up letting it mature, there's that grain, easily ground for pancakes. Fagopyrum esculentum.