A Spanish wine cocktail, literally called "summer red". An Andalusian specialty. I'd been in Spain many times but had never encountered it until I read about it in my Lonely Planet guide and decided to order one. It turned out to be very refreshing and tasty. Tinto de verano is a good way to enjoy wine without getting drunk too quickly and while still having a cold drink.
Although the effect will probably not be quite the same if you're not sitting on a sunny square in Sevilla, it's easy to make this at home: mix equal amounts of red wine and 7up or a similar lemon/lime carbonated drink.
Andromache01 says that Spaniards tend to use cheap wine and soda water. I'd say, experiment and see what you prefer.
Sources on the internet say you can use either a good wine or a mediocre one, but I'd say that a very good wine should be appreciated by itself and not diluted with soft drinks... so go for the cheap kind! Spanish wine is best for this, of course. Add a splash of Martini Rosso if you like that stuff. Add ice cubes and a wedge of lemon. Sit somewhere in the sun. Sip. Imagine you're in Spain. Paradise...