Ammonia fuming is an old and venerable technique. It was first noticed when oak in barns with lots animal manure (yielding ammonia) was noticed to darken much more quickly than the same oak not kept in a barn.
The method will work with any wood, but works better the more tannin the wood has it is works by oxidizing the tannins. In a sense it "rapidly ages" the appearance of wood, that is with time it'll turn dark anyway. Ammonia just (greatly!) speeds up this process.
For woods that do not contain a large amount of tannis these can be added per the tea recipe.
The method is simple. In an airtight container - a properly covred aquarium or even a large airtight plastic bag place the wood to be darkened. Using a plastic plate pour some 20-25% concentrated ammonia solution into the plate - put it in the container with the wood and leave it. Within an hour the wood will begin to darken but the most dramatic results are obtained after 24 horus exposure to the ammonia. Finish with oil or wax.
Note that ammonia is pretty nasty stuff. If you get it in your eye you'll really hate life (ask me how I know) so wear eye protection at all times when handling ammonia. The fumes are very nasy and extremely water soluable, you see ammonia is a gas and the ammonia solution you buy is just this gas dissolved in water. When it fumes the gas comes out of silution and is very attracted to water. Like, your eyes.
If you do a lot of this or do it in a less than optimally ventilated place then you should seriously consider a military gas mask.