Drum stick:

credit: http://alibaba.com

Dishes:
Curries and sambar

credit: eRecipeGuide | Everybody Can Cook
One should not eat its skin.
Availability: Available in frozen section at Indian grocery stores. Haven't seen them fresh.

Elephant foot yam:

credit: mouthwateringfoodrecipes.blogspot.com

Dishes:
Fries and toasts:

credit: euphorhea.blogspot.com
Availability: Haven't seen them at all.

Plantain flower:
This flower can be considered as a vegetable. Astringent in taste.

Dishes:
Snacks and side dish.

chickpeas + plantain flower vada (just like falafel balls)
credit: http://penmai.com
Availability: Haven't seen them being sold.

Bitter melon:
Its one of the bitterest vegetable, you will ever taste.

credit: potassiumrichfoods.com
Dishes:
Fries and snacks

credit: lovencook.blogspot.com
Availability: Available in most of the farmers market along with its chinese variant which is less bitter.

Kohlrabi: (Nookal in Tamil)

credit: Wikipedia

Dish:
Curries and side dish

credit: My Cookbook
Availability: May be in local farmers market.

Unripe Fruits:
Unripe Bananas: (tastes astringent)
Greener ones are unripe and are used as side dish.

Dishes:
Snacks and side dish.
Shredded and cooked as a side dish

credit: http://samai.in
Availability: May be in Indian grocery stores.

Unripe Jackfruit:
Nowadays, Its being used as a popular substitute for meat in vegan diets.

credit: RedChillies

Dishes:
Curries and side dish.

credit: Veg Bowl
Availability: Canned unripe jackfruits are available in Asian grocery stores.