chessGMSolve unlimited chess puzzles for free. Every puzzle is a real position with one best line, rated from 100 for the easiest to 3200 for the hardest, and your own puzzle rating goes up and down with how well you solve them, so the next puzzle is always about right for you. Pick a theme such as forks, pins, skewers or mate in 2, or just keep solving, and play through the whole solution on the board afterwards.
There are also a new daily puzzle, the timed Puzzle Rush and Rush Battle against other players.