UICloud collects the best user interface designs and provides a search engine for you to find the best UI elements that you need.
Invision helps you create more realistic-looking prototypes without adding any extra time to your normal prototyping process
Pttrns is the finest collection of design patterns, resources and inspiration.
AdCat.io, is a design tool that allows you to upload a single picture to get perfect-sized, up-to-date ad images for Facebook, Instagram, and Twitter. It is 100% free, no signup is required, and it takes seconds!