One classic game a day, Atari 2600 style, with a single try and one leaderboard for everyone. There are 24 games in the rotation, from Snake and Pac-Man to Boulder Dash and Joust, all written from scratch, and the music is public-domain pieces turned into chiptune.
Qix (Taito, 1981) · Music: Marcha de los enanos (Trolltog), Piezas líricas op. 54 n.º 3, Edvard Grieg, 1891
Carve up the board. You leave your own ground by drawing a line into the open space, and when you step back onto your ground the area you closed becomes yours — but only the half the Qix is NOT in.
While drawing you are defenceless: if the Qix touches your line, that's it. Sparx patrol the border of what you've claimed, round the corners, and cannot enter the open space: they won't follow you there.
Full QIX guide: controls, scoring and tips
Every day brings a different game, with the same map, the same seed and the same music for everyone. Nobody gets an edge: what you get is what everyone else gets.
You have a single run that counts. You can practise any game as often as you like in practice mode, but the daily challenge is played once.
Scores are not taken on trust: the server replays your run keystroke by keystroke with the same game code and only accepts it if it reaches the same result. That is why the leaderboard cannot be faked.
At midnight the day closes and the top 10 is published. First place becomes champion and can leave a line that everyone sees the next day.
Every past challenge, with its podium
SNAKE · BREAKOUT · PAC-MAN · NIBBLER · TETRIS · JEWELS · GALAGA · INVADERS · FROGGER · ASTEROIDS · PANG · DIG DUG · CENTIPEDE · MISSILE CMD · KABOOM! · BATTLE CITY · BASKET · SPACE IMPACT · RAPID ROLL · LUNAR LANDER · JOUST · BOULDER · PIPE MANIA · QIX