I come across tools all the time. Tools to help me build, tools to help me maintain, tools for optimization, tools to measure performance, tools to measure audience or user demand. Below are a few suite of tools I use whenever I'm troubleshooting a bug or trying to improve the performance of a web app.
Is it Down or Just Me? can help you quickly determine if a website is down or maybe it's just your internet connection.
Analyze HTTP Headers - This tool is great for helping you, Full Stack Developer, understand the quality of your HTTP headers and requests.
SSL server tests - This website is great for helping you, DevOps, understand where the vulnerabilities lie on your website.
Crashlytics for crashes - Crashlytics is excellent for keeping a log of how many users are crashing due to technical errors in your app.
Down or Me - This website will help you determine if a website is down or if it's your browser, OS, computer, Wifi connection, etc.
Page load time - This is a Google Chrome extension that gives you a very neutral page load reading.