Startup School - How to start a startup, by Y Combinator
YC Interview Practice App - Get asked YC questions by Sam Altman
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.
interviewing.io - Free, anonymous technical interview practice