Very soft inside, small seeds, tastes bit sweet
It looks like either Pepino Melon (which I'm growing this year, and I tried to grow in 2015) or Tzimbalo, by the color, the dark striping and the leaves. Eggplant is related to both, but eggplant leaves leaves are broader and bigger than I see in the picture, usually (if not always). Pepino Melons are typically (not always) bigger and longer than Tzimbalo, but it's hard to tell from your picture which it is. I personally think it looks more like a Pepino Melon by the color, lack of sheen, and the way the leaves look.
It looks like the plant might have spider mites, by the speckling on the foliage. Mine had spider mites in 2015. If you do have spider mites, showering the plant with a shower nozzle on your hose every two or three days may help, at least if you're in a semi-arid to arid climate.
Pepino Melons are not melons, although some people think they taste like them. They're in the Solanum genus of the nightshade family.
Looks like a small eggplant (Aubergine for you Brits), possibly not yet ripe. (Credit to Huangism, who said so first in a comment.) Botanically, it's a fruit (and a nightshade!!); culinarily a vegetable.
Note that the leaves of eggplants varies considerably. Here's a page (with a photo showing both leaves and a 'fruit'), where the leaves are IMHO fairly similar to those in the Questioner's photo: http://www.learn2grow.com/plants/solanum-melongena-listada-de-gandia-images-large-20148/
IMHO wonderful in Caponata; here's a recipe to try: http://theitaliandishblog.com/imported-20090913150324/2012/9/11/sicilian-caponata.html
Yes. I found out it is pepino melon!
To me it looks like a Mexican tomato. Tomatillos A native plant to Mexico. Tropical or warm weather plant.