Diet Coke is a wonderful drink. Not only is it full of caffeine and low in calories, but it has many other useful properties. Its lack of sugar and real caramel, combined with its slightly acidic nature, makes it a decent cleaning solution in a pinch. I've used it to clean mouse pads and other plastic/glass surfaces with some success. I've even heard of people cleaning circuit boards and keyboards in it, but take that with a grain of salt!
The carbonation seems to make Diet Coke very effective at scrubbing out dirt from most kinds of artificial surfaces. I wouldn't recommend it on porous entities or clothes simply because it does contain some coloration that can stain if it soaks in.
Another use for Diet Coke is as a mouthwash (perhaps not the most pleasant, but still better than morning breath).