IAF and Lichess.org
Lichess.org is a popular free and open-source Internet chess server run by a non-profit organization. Lichess is ad-free and all the features are available for free, as the site is funded by donations from patrons. Besides classic chess, players can play certain number of chess variants, including Antichess.
We mainly use Lichess as a platform to play games and to organize tournaments. The players ranking and rating are also based on the rating on Lichess.
However, it is important to highlight that we are not in any way affiliated with Lichess.