Startup School - How to start a startup, by Y Combinator
The PM Interview - Practice PM Interview Questions (w/ timer).
Apply to YC (Y Combinator) - Curated collection of resources for founders applying to YC.
Pramp for Product Managers - Practice product management mock interviews with other PMs
YC World - Explore Y Combinator companies by country
Myskillwall - The best tool to prepare your job interview