Ask HN: What tools do you use to improve your product?

Until recently, I had only been using Google Analytics for tracking usage and seeing where I need to focus my attention for improving my site.

I recently started using Sentry.io for tracking errors and Hotjar for getting better, in-depth user interaction data. Now I'm questioning why I waited so long to sign up for these kinds of products.

What tools or products do you use to help you improve your product?