Interfacer - Collection of more than 200+ free design resources
Brand.ai - Create a style guide & integrate with Sketch, Slack, & Gulp
Facebook Design Resources - A collection of free resources made by designers at Facebook
MediaKits - The easiest way to create a media kit
Freebiesbug - Collection of the best free web design resources.
Frontify Workspace - The overall tool for creative & design collaboration: - upload, manage & share assets