SuccessRate - You are welcome to submit your draft application before a deadline to get alumni feedback.
The Y Combinator Database - The definitive database of YC companies with all the metrics
YC Interview Practice App - Get asked YC questions by Sam Altman
Y Combinator Companies - Interactive list of all 1000+ YC companies
Reject Them Softly - Rejection letter inspirations from companies
Startup School - How to start a startup, by Y Combinator