Startup School - How to start a startup, by Y Combinator
Pramp for Product Managers - Practice product management mock interviews with other PMs
SkySilk - SkySilk Managed Cloud Services, Offers Free Cloud Resources
The PM Interview - Practice PM Interview Questions (w/ timer).
Apply to YC (Y Combinator) - Curated collection of resources for founders applying to YC.
Speechapp - The simplest way to practice for your next job interview