What to grow kind of depends on what you can find relatively easily, for example I can get seed for Field, or Winter Rye easily while seed for a winter wheat is a little more difficult. Talk with nearby garden, or feed, stores to see what they carry. Stay clear of Annual Ryegrass, a weedy grass often used as a nurse crop for lawns.

I am a big fan of winter rye. It makes a lot of growth below soil too. I till it under when about 4 feet tall, then when it has decayed some I put in buckwheat and till under when at about 80% bloom. I then put it back to rye for the winter, or broadcast oats which will die at frost yet leave a cover on the soil. I have enuff land to alternate years of production crops.

I like winter rye also. Stays nice and green all winter when everything else
turns brown. It reall seems to help suppress weeds the following year. Alfalfa I've used only when you're gonna give it a full year.

I've been looking at crimson clover because its a legume and it can reseed itself so I can just leave it in the areas I don't have crops growing as a living mulch and wanted to know if this would be a good idea. It says it is successful in the SE and some of the SE is zone 7 like myself. Anyone used crimson clover?

I just bought a mix from Johnny's Seeds that contains winter rye, field peas, crimson clover and hairy vetch. This will be my first year growing a cover crop, so I thought I'd try the shotgun approach.

If you seed your cover crop into the soil in the fall with plants to plant in that bed the following spring seeding a legume of any kind is kind of a waste since the Nitrogen nodules will not have developed by the time you want to plant, it takes almost a full growing season for that to happen.

With winter rye, would I be able to collect the seed to use for the next year or would I have to buy more for every season?

You can save it, every year for many years. Sometimes I brush hog it down when it is ripe and it will reseed itself, even without mowing it will reseed.