Tannic acid when you want it.
For field work where we didn't have tannic acid handy we brew up a six bagger pot of tea and steep to the point of blackness. The resulting brew did quite nicely for preserving archaeological iron artefacts.
I would recommend loose tea as the tea bag often contributes to the brew. Unless you really want a staining developer I would use distilled water or at least reverse osmosis water and not well water for instance. Even city/house water can have appreciable lime and ferrous ions as well as curpous.
Tannic Acid Safety (small note_
I had already posted and then remember something I read a while back. Under normal useage tannic acid ought to be just about the safest developer possible, I think. However, there is one very special case where it is deadly or can be. In the late 1930's there was quite a stir in the medical community for treating massive burn victims. It was found that tannic acid powder applied to the burned areas formed a breathable protective mass and kept the area clean. People actually began to recover in situations usually never possible. But, then they died. Apparently, tannic acid in the blood and carried to the liver, damages and kills the liver when present in sufficient concentration.
What a wonderful thing the stomach and GI tract is, eh? (I am Canadian so the eh is obligatory) Bio-evolution wins again and keeps us in a tasty beverage.
I can't see that ever happening when used in photography however it is when we become too trusting that accidents happen and I would want to use gloves if I had large areas of abraded skin on my hands or open wounds and sores.