I recently found a line of all-natural bottled sodas at Whole Foods called DRY Soda that are relatively low calorie and pretty tasty. The flavor I like is called Lemongrass (which is subtly sweet in a really refreshing way), but the soda also comes in Vanilla Bean, Juniper Berry, Lavender, Kumquat and Rhubarb. The sodas vary from 50-70 calories per bottle (which are 12 ounces so 1.5 cups), depending on which flavor you buy.
As I mentioned, I found this soda at Whole Foods, but I read on their website that it is also available at Bev Mo as well as various other retailers.
PS. In the non-water drink department, I'm also a big fan of low calorie Izze Esque soda, Mint water, and Fizzit, as well as making my own fruity flavored water using real fruit like they do at the spa.