The ones Trish is talking about - the ones with no valve? Those are the ones that survived Ella's rocket launching with nary a drop of leaked liquid. I started both kids with a Nuby because of the soft tip which made transition easier (not easy, maybe I should say less traumatic - for me, because they both hated sippies at first), but they leak. When you look for it in the store, it will note on the package that it has a built-in valve, and it's the only one I've seen like that. They have them at Target and BRU also.
Okay, I just found a link! It's by The First Year: http://www.mommafindings.com/2008/10/first-years-sippy-cup.html
I have no idea what this site is, but it has a photo of the sippies and isn't a commercial site, so I picked this one from the google search :).