I've been using the olive eyeliner from Earth's Beauty for several years now and am very happy with it. It's nice and soft, so easily blendable, causes no irritation and lasts well all day. The colour is very neutral and it just helps to define my eyes nicely.
Many natural eye liners in this sort of shade contain carmine, which is derived from insects. This formula doesn't, so it's a great option for vegetarians.