The Y Combinator Database - The definitive database of YC companies with all the metrics
SE Ranking - All-in-One cloud-based SEO software for agencies, marketing teams and webmasters
Y Combinator Companies - Interactive list of all 1000+ YC companies
DeskAlerts - Send alerts to any corporate device: PCs, phones, tablets, etc.
Startup School - How to start a startup, by Y Combinator
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.