I have been drinking tulsi (holy basil) tea on and off for years. It has a calming and stress-relieving effect. Maybe not instantly with one cup. But if I was in a chronic high stress state and drank multiple cups a day for a week or so, it made me continuously more relaxed.

Also tried capsules, but funnily enough I didn't get the same effect as with tea, although I'd think the extract in the capsules is more potent.

It's also a natural anti-histamine.

Edit:
If someone tries the tea, the quality of the different products is pretty hit-and-miss. The flavor can also be pretty different. A few bad products look like someone has picked dirty weeds from the side of the road and put them in a bag. I have had the best experience with tea bags from 'Lebensbaum', but I don't know if that's available in other parts of the world. On iherb you can find multiple products in capsule form.