Trailblazer is a high-level Architecture libraries for the Ruby web application (not only for Rails). If you are no familiar with it yet - take a 20 minutes walk through guide...
I want to cover once again those places where our team had issues or misunderstanding. Trailblazer documentation got a lot of improvements recently and keep getting more and more care. Some points from this list are covered by the docs, but in practice not everything was smooth.
How to install and copy CSS/JS resources via Composer using Composer plugin.
Flysystem - a filesystem abstraction which allows you to swap a local filesystem with a remote one. That could be very useful for assets manipulation system in CMS or something.
libphonenumber for PHP - a library for parsing, formatting, storing and validating international phone numbers.
Geocoding PHP library with a huge set of geocoding services abstraction.
Firefox Nightly (version 29) supports CSS variables. Okay, now give native CSS inheritance and I will be happy :)