Cover crops, often called green manure, are a great idea; they feed the soil, suppress weeds and prevent run off. The problem with cover crops is they need to be planted while it is warm enough for the seed to germinate and that can mean giving up growing space while your vegetables are still producing. One compromise is to plant it under your cool weather vegetables, but at some point, you are going to need pull the vegetables, to turn the cover crop under. It's worth it though. Peg Tillery, of WSU Kitsap Extension, fills us in on her personal cover crop preferences for her Northwest garden and Michael Bomford, from Organic Kentucky, goes into even more detail about the choices and uses of cover crop grains and legumes.
Another option, especially for those of us who procrastinate until frost arrives, is to dormant seed a cover crop. Genevieve Slocum with Dave Wilson, at the Rodale Institute, say it's (Almost) never too late to plant hairy vetch. If you can get the seed down before the soil freezes, you could have a lovely cover crop ready to turn under for the spring season.