What are some popular Connect 4 variations that add new twists and challenges to the classic game?

Some popular Connect 4 variations that add new twists and challenges to the classic game include Connect 5, where players aim to connect five discs in a row instead of four, and PopOut Connect 4, where players can remove their own discs from the board to disrupt their opponent's strategy. Another variation is Connect 4x4, which is played on a larger board with four rows and four columns, adding complexity to the game.