I am looking for an advanced set of puzzles and the ability to track progress, similar to chessok (which looks good, but is not free, and the selection of puzzles is limited compared to their offline courses). My skill level is >2000 ELO. Are there any alternatives to Chessok?
ChessTempo allows for unlimited solving of tactics problems with a free membership. It has a large database of problems drawn from games, and it tracks your solving statistics along with your individual tactics rating. Also, you can track your progress on two sorts of problems: untimed, or where the elapsed time for solving factors into your performance rating.
Chess Tactics Server is quite competitive. It measures your performance based not only on the correctness of your solution, but also how long it took you to solve the problem. It maintains a leaderboard, so you can see where you stand in the whole world (among the active players) as well as in your own country.