restc is an HTTP server middleware, aiming to enhance debugging experience on RESTful APIs.

debugs memory leak of running process, without recompiling or restarting, on GNU/Linux and FreeBSD

Web Debugging Proxy based on Chrome DevTools Network panel.

An open source, extensible, platform-agnostic tool for remotely debugging and testing your JavaScript. Powered by node.js and socket.io.

MailCatcher runs a super simple SMTP server which catches any message sent to it to display in a web interface. Run mailcatcher, set your favourite app to deliver to smtp://127.0.0.1:1025 instead of your default SMTP server, then check out http://127.0.0.1:1080 to see the mail that's arrived so far.

jQuery-inlog allows you to easily debug your jQuery code. See exactly what you pass to functions, what they return and in which context (this-value) they were executed.

The DebugBar integrates easily in any projects and can display profiling data from any part of your application. It comes built-in with data collectors for standard PHP features and popular projects.

Helium is a tool for discovering unused CSS across many pages on a web site. The tool is javascript-based and runs from the browser.

whoops is a nice little library that helps you develop and maintain your projects better, by helping you deal with errors and exceptions in a less painful way.

Holmes is stand-alone diagnostic CSS stylesheet that can highlight potentially invalid or erroneous HTML(5) markup by adding one class — Read more