The App is called "Lines" (Sometimes DL Lines or Disneyland Lines). It works because it's real data, not a rehash of the Posted Wait times (which people should know are never, ever correct, especially when Disney "stacks" the line (makes them seem longer than they are to get people to distribute more throughout the park...)
Unless somethine weird has happened... I find that it's correct...as in dead on over 90% of the time. I've had lines that said "45 mins" ont he outside with my group and Lines says 17 minutes... I tell my group to stick around... they moan... I start the timer and just before getting on the ride.... 17 minutes waited! Almost every time!
I Tried Mouse Wait a few times, and really didn't get much out of it other than the reports of a ride down... MouseWait seems to get more people posting, but again, they just post the posted time on the ride, which Lines will prove to you is basically worthless. (They tell you too, but it's amazing when you see "Wait Time Posted" and "Wait Time Actual" so far apart some times.