I'm not a huge advocate of supplements. As Vinnie Tortorich says, "taking lots of supplements will just make you have the most expensive pee in the world."
But there are a few that I have found some benefits from:
Magnesium -- I get the Natural Vitality powder and take about 2 tsp every night dissolved in water. I initially did this to help with my rotator cuff but found that it really relaxes me and puts me to sleep at night. It's also a supplement that Phinney and Volek suggest using for low card lifestyles.
Sodium -- important to supplement with this on a low-carb lifestyle since your kidneys excrete sodium at a higher level. I use veggie bouillon cubes that just have salt, veggies and palm oil.
Apple Cider Vinegar -- every morning when I wake up, I put about a tsp of ACV in 6-7 oz of water and drink up. It's supposed to alkalize the body. Lots of benefits of ACV other than that, too. It's cheap.
Multi-Vitamin -- I have a multi vitamin that I take when I think about it. Not every day. Not religious about it. I could take it or leave it.
Gymnema Sylvestre - click the link. It helps to lower my blood sugar (or keep it from rising very much) after meals. I get the NOW brand.