I had a good experience being part of The Food Project CSA http://thefoodproject.org/ for the past couple of years. I split it with a friend (we alternated weeks), so the cost felt reasonable and we liked their mission. Our pickup was over on School St. so that's really closer to Union, but might be far if you're car-less from Davis.

I can't personally vouch for any others, but a friend of mine was part of the Enterprise CSA for awhile and seemed to like it. They have a pickup at Kick@ss Cupcakes.