Ran across this machine (http://www.sodaclubusa.com/prodinfo.htm) on Gizmodo. For $100-130, the device makes Coke, fizzy water, or whatever you want for 25 cents per liter.

Might be fun to make sparkling, 100% fresh fruit juices with it, or other creative options. They sell pre-made knock-off packets of Dr Pepper & Mountain Dew that use Splenda, too.

I'd buy it, but I fear I'd be too lazy to use it more than once. The CO2 bottles are $18 per 110 liters of water.