The Y Combinator Database - The definitive database of YC companies with all the metrics
Startup Toolkit - Interactive how-to guides to launch your startup
Startup School - How to start a startup, by Y Combinator
Techstars Entrepreneur's Toolkit - Online resource to help you learn about entrepreneurship
Y Combinator Companies - Interactive list of all 1000+ YC companies
AwesomeHunt - Community-curated lists of learning resources on any topic.