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Lichess.org is fun too. I'm not much of a chess player but I sometimes find the chess puzzles to be fun at http://en.lichess.org/training

That is really useful, I like assortments of puzzles like these. They keep my brain sharp.

http://chesstempo.com is the state of the art for tactics puzzles auto-adjusted to your strength.

And for the ability to review already seen problems using spaced repetition.