The Binding of Isaac is one of the most expanded indie games in history. It's had many, many updates, some big DLC releases, entire code overhauls, ports to various consoles and platforms... and apparently that still wasn't enough for some. Taking the theme of the game and running with it, fans have created a tonne of new monsters, bosses, items and unlocks for players, bundling them all together into one expansion called Antibirth.
Antibirth adds a lot of new content to the game, but in a nutshell players can expect to find:
Two new characters to play with.
More than 90 new collectible items.
18+ more trinkets to unlock for additional strategy.
A massive 16 more bosses to take on, with varied ways to be killed by and to kill them.
80 new monsters to face off throughout the various rooms in the basement.
4000+ new rooms to explore.
A new secret final chapter for Isaac to take on.
Better local coop play.
A brand new soundtrack, that's evocative of the original Isaac theme.
That's a hell of a lot to work with and work through. It should keep fans happy for a few more months before the undoubtedly next big update is released.