Chess Tactics Server
Recently got back into chess tactics training. I highly recommend this site to improve your chess tactics, especially if you’re a beginner or intermediate (who at least has some grasp of how to play). It tracks your progress, and the rating gives you incentive to continue a training session (e.g. ‘I must get over the 1500 hump before I can leave the computer’). For anyone who doesn’t know what tactics are: https://en.wikipedia.org/wiki/Chess_tactic For low-level and intermediate-level play, solving a ton of chess tactics problems is generally the fastest way to improve your chess performance. And it’s healthy for chess players of all levels (akin to running for soccer players).
If Chess Tactics Server is ever down, Chess Tempo is a strong temporary replacement. http://chesstempo.com/