Denby described this Half-Life 2 total conversion as "totally and utterly rubbish, and one of my favourite mods so far this year" which, when paired with its name, pretty much guaranteed I'd go take a look. You're probably expecting it to be a game about shooting zombified animals. You'd be wrong. It's a game about punching zombified animals.
Denby's right. Zombie Zoo is rubbish, and it is delightful. It's quite clearly aware it's rubbish, which gives it free license to be even more rubbish. If you play along with this mentality, you'll be thoroughly entertained. This is a game where you can punch brains around a car park, or punch lurid green zombie rabbits that barely move, or punch a lurid red boss zombie rabbit that moves rather fast, or... well, more punching, mostly. Also jumping, and crate-pushing, and being killed by bear traps with 5-metre-wide activation radiuses.
Zombie Zoo doesn't even slightly try to be interesting in terms of game mechanics - this is late-90s levels of 3D crate-pushing. It is, however, set in a zombie zoo. That counts for a lot.
Jurassic Park seems to be the thematic influence, with a semi-kindly businessman narrator directing you, as an unnamed intern, around the inevitably disaster-struck menagerie. Only there's another guy there too, who threatens and cajoles you. That he sounds just like the other dude putting on a husky voice is probably of zero import.
There's a touch of Portal to the narration - absurdity meets uncertain menace - but mostly it's panto. Panto with zombie bears and neon whack-a-rabbit. Small-but-silly details/gags abound, and that's really where the fun lies.
Just the introduction and first level so far, but I'll definitely be playing the rest as and when it appears. Even though it's rubbish.