During the growing seasons, they forage for themselves. Once there's nothing green available anymore, I add sprouts and wheat grass to their food plan. There are lots of threads on sprouting and growing wheat grass. There are also a lot of web sites on sprouting and wheat grass.
Alfalfa is good, too, but they usually like the leafy part, rather than the stems. Some people soak the pressed cubes, but my chickens never liked those. I think some people have fed plain alfalfa pellets, too. You can even feed alfalfa sprouts, although that's more expensive. When I sprout alfalfa for myself, I usually do an extra tray for the chickens. They do love them.
I basically just try to supplement their diet in the winter. I know they aren't eating as well as in the middle of summer, but it's better than not supplementing at all. I also try to add some non-soy protein to their winter diet, since they aren't getting all the bugs and worms from foraging during the winter. Plus they still get some sunflower seed along with their winter scratch.