The Happy Zombie Sunrise Home
Over on Wattpad, two very distinguished authors are creating a serial zombie novel together. The serial is called The Happy Zombie Sunrise Home. Margaret Atwood and Naomi Alderman are a mentor/protege team who write alternate chapters of the narrative.
Twilight of the Dead
Travis Adkins has his serial version of Twilight of the Dead on his website. Click the red "Read For Free!" link.
Dredging Up Memories
AJ Brown has a serial called Dredging up Memories posted at Tales of the Zombie War. There's more about AJ Brown on Julianne Snow's blog: Days with the Undead
David Wellington has his novel Plague Zone available in chapters at his website. It's also available complete on Kindle.