Startup School - How to start a startup, by Y Combinator
SocialSizes - Sketch templates for sharing images & videos on social media
Apply to YC (Y Combinator) - Curated collection of resources for founders applying to YC.
Invision - Prototyping and collaboration for design teams
YC World - Explore Y Combinator companies by country
Craftwork - A collection of User Interface resources made by Craftwork