Or there's this relatively new thing called the "Sodastream" which has been advertised a lot on the radio lately....
http://www.sodastreamusa.com/
I can't personally endorse it, since I've pretty much stopped drinking carbonated sugar drinks of any kind on a regular basis, but it appears the flavor syrups made by the company are sugar and not corn poison, which is good. And the diet flavors are Splenda rather than the Donald Rumsfeld neurotoxin aspartame. Which is better, but not the best, which would be Stevia.
But there's no rule that says you have to use their syrups at all. You could probably just as easily carbonate your own juice or lemonade or whatever.