Anyone have a favorite "greens" supplement?

I eat pretty well and am good about AT LEAST 5 servings of fruits and veggies a day. But I've been hearing about greens lately and wanted to try them. I get so tired in the afternoon (3 small children will do that to a person) and I wondered if that could help my energy level. I'd also like to have an immune system on steroids this winter.

Any favorite brand or a recommendation?

Thanks!!