I had this it a local raw restaurant - and subsequently duplicated it at home. Simple but surprisingly refreshing - if you like ginger. In the past when I've made homemade ginger ale, I've cooked down a sugar syrup with ginger. Really, this isn't necessary and actually takes away some of the nice ginger bite. Instead...
Put some fresh ginger through a juicer. Or, if you don't have a juicer, finely grate the ginger and press out juice through a fine strainer.
Stir some of the ginger juice with soda water and light agave nectar as a sweetener (sure you could use regular sugar or sugar syrup, but the agave is nice and "low glycemic"). The ginger juice is powerful -don't add too much! Experiment with proportions that you like.
Serve in a tall glass filled with ice cubes and a slice of lemon. A very nice, refreshing ginger ale.
You can also add fresh lemon or lime juice, of course, but I kind of like the purity of ginger on its own.