Skype. It used to actually be a pretty capable app for making/receiving calls and instant messaging. Version 2.8 was pretty solid. That was right before they started messing it up.
Remember v2.8? It has the little unobtrusive contact list. It behaved like an instant messenger, but it also made phone calls. It just worked.
Then Version 5 came out. They went for this "full screen" UI with an attached contact list. You now see that same design throughout the (awful) mobile apps. Skype has basically just become bloated and heavy.
I'm actually still using v2.8 and refuse to update. But I'm stuck with the terrible mobile apps.
This leads me to my question: today, with their app spread across so many platforms, how can they fix it?