iOS: Most to-do apps that allow you to collaborate with people require everyone to sign up for an account. List Weaver keeps things simple by integrating with iCloud so sharing lists is incredibly easy. It's also free.
With List Weaver, you share a to-do list through AirDrop or email, and that list is then automatically updated when someone makes a change to the file. It's incredibly simple, and as a means to easily collaborate on a list it delivers. List Weaver doesn't offer a bunch of customisations or even delegation tools, but as a tool to easily collaborate on lists it works well.