Concha y Toro is an old estate house surrounded by vineyards, tucked away at the bottom of the foothills of the Andes. It's famous for having been the discovery place of a grape that was believed to be extinct, after it was killed by disease in Europe. But a few plants survived, sheltered in Chile, and now you can try them at Concha y Toro.
I visited with a group of fellow English teachers and we took the basic tour, which costs around $15 and includes two wine tastings and a souvenir wine glass. We got to see the vineyards and the wine cellars, and there was a fun story about the "devil in the wine cellar", according to which a devil protects the wine from any would-be thieves.
If you find yourself in Santiago, I definitely recommend a trip to Concha y Toro. There are some other less tourist-y places as well, but for something easily accessible from the city and a good quality tour, it's a great escape from Santiago.