Kokum

Kokum is a fruit related to the mangosteen, and it is native to India. The dried fruit is used as a wonderfully fruity souring agent, and it works particularly well with pulses and vegetable curries.
Just drop a piece into the curry during cooking and let it infuse, resulting in a tartness that matches really well with the other spices in the dish.
It also works brilliantly in fish curries, when its sweet/sour nature comes into its own. Due to its sour taste, kokum can be used as an alternative to tamarind.