I bought some spirulina tablets ages ago for their nutritional value. They're supposed to be absolutely brilliant for you. They tasted awful and I really didn't notice any difference to my overall energy or wellbeing after taking them. I've toyed with the idea of buying some in powdered form because apparently you can add it to smoothies for an extra nutritional boost, but I never quite got around to it.
If you're keen to give it a go buy a small amount first.