Are there any others?
Edit (25.12.2020): Since at least 2018, chess.emrald.net has often not been responsive.
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Free: 5 problems/day
Lichess.org has both tactics and opening training.
chessity.com is fun, and the free membership offers 20 puzzles/day and access to multiplayer games.
Take a look at this.This chessbase site is pretty useful.