Startup School - How to start a startup, by Y Combinator
YC World - Explore Y Combinator companies by country
Co-founder's Daily Journal - One of the key issues founders are struggling with is co-founder conflict.
Y Combinator Companies - Interactive list of all 1000+ YC companies
Co-founder Question Cards - Questions to ask your co-founder to know each other better.
Apply to YC (Y Combinator) - Curated collection of resources for founders applying to YC.