A free app for Android, by Play Magnus.

Tactics Frenzy: Chess Puzzles is an innovative app that has been developed by the world champion in chess, Magnus Carlsen, and his team. In this game, you can solve tons of unique, entertaining, and high-quality chess puzzles. You can choose the level of difficulty, which will determine your time and effort requirements.

Some of the puzzles had been generated by computer, while a few man-made ones were eventually thrown into the mix. All of these are pre-approved by the team of developers. Download it now and have fun!