ChessQuest for iOS
This is one of the first chess apps I bought soon after the iPhone came out. As far as I knew, it was the only app of it's kind. It offers intermediate/advanced tactics in a very simple format. The problem selection is great. For a while it disappeared off the app store. Now it is back (and it currently has a 4/5 rating). Now there are many apps that ofter training puzzles. Most of them advertise that they have thousands of puzzles with more added daily. ChessQuest only have a couple hundred (you might want to follow up on this number) but they are high quality and interesting positions.