What are the most essential tools for new tech entrepreneurs?

Global leader in #ProdMgmt training. 1M+ community. Instructors are top PMs working @ Google, Netflix, Uber & Airbnb. For entrepreneurs just starting out with limited budgets, what are the best app analytics tools you use?. This Slogging thread by Yuvraj Malik, Akshay M. Bharadwaj, Will Fang, Jiani wei, Evan Leong...