Intercom is keeping us awesome at support and user engagement.
We use FoA for the grid and as the base styles for some of our components (e.g. buttons, cards).
We use bower for keeping our packages well organized and updated.
Grunt helps us keep build and minify our code for production.
Browserstack is a lifesaver for IE and quick mobile testing.
For crushing code!
Sass is being used to keep our stylesheets organized in addition to the typical use cases (variables, mixins, functions, etc).