I've stopped drinking diet or zero anything, because of the aspartame. It's been linked to pre-term birth in a recent study so I figure the risk just isn't worth it for me, for the sake of avoiding a few calories. Most low calorie drinks, including a lot of flavoured waters, contain it - check the labels!
I still drink full-fat coke and dr pepper though, and fresh orange and lemonade if I go out. All Asda's own brand stuff is aspartame-free as well which was good to know - they use sucralose instead which I think is one of the slightly less evil artificial sweeteners (I think aspartame is the king nasty and acesulfame is not far off)
Mostly I just drink water though. Managed to kick my Diet Coke habit when I found out I was pregnant and am keen not to replace it!