I've tried a cranberry flavored water (Fit & Active, from ALDI'S) that tasted just like cranberry juice--minus all those calories and carbs. I haven't tried mixing it yet, but I think it would be a great subsitute for cranberry juice and vodka.
I've tried mixing vodka with a diet orange soda and a diet fruit punch before. Both were pretty good. Before I was low carb, I mixed light or diet "juices" (V8 Splash and Minute Maid brands) with vodka. These were good too, but do have a few carbs (about 3) per serving.
I'm personally a beer fan, so haven't come up with any creative mixes yet! Now that I'm carb conscious, however, I think I'm gonna start, so thanks for all the great ideas!