Infinite chess is a variant where the board expands infinitely in all directions. It can lead to some funny moments by moving your queen really far away than moving around and attacking the king from millions of squares away. It can be found by searching up "infinite chess" on Google. It is not a variant on chess.com or lichess.org.

Atomic Chess

A variant where capturing a piece causes every piece adjacent to it, except for pawns, to also be taken, even your own pieces. This also applies to the king so be careful about pieces right next to your king. This variant is available on chess.com and lichess.org

Fog of War

A variant where all your opponent's pieces are invisible to you. Any pieces occupying the squares that your pieces can move to will be visible. This forces you to make sure your king isn't exposed or else your king might be captured without you knowing. This variant is available on chess.com.

Three-Check