I needed a wood surface treatment for a few projects. I wanted a natural look but didn't like any of the tiny jars of wood stain that were collecting dust in the basement. A quick search yielded a recipe using things I already had around the house, didn't involve nasty chemicals, and sounded more permanent than other options that could be rubbed or scratched off.
There are a lot of sources for this so I won't cite any particular one, but the basic premise is this:
mix a quantity of vinegar with a quantity of steel wool (the finer the better), let sit for at least 24 hours
brew some dark tea (I used Earl Gray I think), brush onto your wood surface, allow to dry
brush the vinegar and steel wool solution onto the wood
call it a day
I used this method on cheap pine boards from the local hardware store and on some fancier woods all with nice results. If you're curious as to what's happening, here's the jist...
Vinegar easily dissolves the fine fibers of #0000 steel wool, making the iron within available in the final solution. The longer you let it sit the more concentrated the solution. I found that leaving a little bit of steel wool in vinegar for a few days completely dissolves it.
The vinegar/steel wool solution reacts with the tannins in the wood to turn the wood fibers a darker color. The shades depend on both the concentration of the iron solution and the amount of tannins in the wood.
Treating your wood with black tea, which is high in tannic acid, helps create a more pronounced reaction in woods (like cheap pine) which normally would have a mild reaction.
You'll obviously want do some tests before applying this method your project. What I really like about this method is that it holds up better than simple surface treatments, as it's a legit chemical reaction occurring within the wood fibers themselves instead of just on the surface, and I think overall gives it a more natural look.