NoRedInk/elm-init-scripts - I don't use these but it's a nice insight into how NoRedInk structures their Elm apps internally.
json-to-elm - A tool from NoRedInk that makes it easy to build Decoders for a given JSON structure, either in the Decode style or in the elm-decode-pipeline style.
The NoRedInk Elm Style Guide - A living document describing how NoRedInk writes their Elm code. This is updated pretty regularly and it's got lots of great tips in it.
Basics - Lots of useful functions that are imported by default.
Composing Features and Behaviours in the Elm Architecture - A really comprehensive multi-part article on how to build composable features in Elm. I haven't yet had a chance to explore any of the ideas covered here, but I've heard good things about it. Worth noting that this was written in the 0.16 days though.