@Mor - I wonder if they ever tried to get tannins from the redwoods back in the days when they still cut them down regularly? I don't think tannic acid is used in the commercial tanning of leather these days, but back then it probably was.
Actually I think these days most tannic acid uses are medical. I know they used to use it to treat burn victims. I've also heard that you can use cold tea bags to stop bleeding in your mouth (after going to the dentist for example) because of the tannic acid in the tea.
It's one of those remedies that's pretty good as a home remedy, but in general there are other things discovered by modern technology that will do better if you get them from the doctor.