Hansen's diet sodas are sweetened with Splenda. They are caffeine free and come in several flavors--I love the ginger ale but there is also kiwi strawberry, tangerine lime, etc.
I will come right out and admit that I sometimes drink a 6-pack a day. I used to be a regular Coke addict and I am still hooked on the sensation of sweet, cold, and fizzy. I have never felt that they have hampered my weight loss or inspired sweet cravings (if I crave sweet, I just have another one.

), and I lost 60 lbs in 2002. (Gained 30 of 'em back in 2003, not because of diet soda, now trying to drop 15 permanently as a happy medium.) The only adverse effects of diet soda I have noticed are that, because you drink air along with liquid, they can temporarily make you look or feel bloated.
Too much artificial sweetener is not optimal, I agree, and if I were pregnant or breastfeeding I would quit, but until then the amount that diet soda helps me stay on a low-cal, low-carb plan outweighs the possible risks.