Lichess
Description
Lichess is a free, open-source chess platform powered by volunteers and donations.
In it, you can play the traditional chess game online with the StockFish chess AI (toned down to different levels to make it actually possible to win), random players or friends, as well the variant gamemodes such as anti-chess, crazyhouse, horde mode and others. You can also train your chess tactics with randomized puzzles.