Startup School - How to start a startup, by Y Combinator
Startup Toolkit - Interactive how-to guides to launch your startup
Apply to YC (Y Combinator) - Curated collection of resources for founders applying to YC.
Techstars Entrepreneur's Toolkit - Online resource to help you learn about entrepreneurship
YC World - Explore Y Combinator companies by country
App Launch Checklist - Planning tool for launching your app successfully