PL/SQL Instrumentation With LOGGER
LOGGER is another open source tool and we have used it for years. Logger is a logging and debugging tool that lets you monitor the progressing of your pl/sql code. At its simplest level, you can log process messages and errors that help you to both document your code and track and log exceptions. At a more advanced level, Logger will help you monitor and log the timings of each section of your pl/sql procedures.
Logger has a number of features specifically for Apex. For instance, you can log all session variables in order to better monitor and identify issues in your Apex application.
Logger is free and easy to install and use.
APEX-SERT : Applying Apex Security Settings Was Never Easier
This is a really cool and powerful open source tool. It is the very best way to correct and learn the security setting of Oracle Apex.
APEX-SERT audits an application and checks the security risks for all pages and at the application level. It provides an explaination of the vulnerability and how to correct it. As you make these changes your application security score will update according to the changes that you have made.
This tool is also perfect to use throughout the development of your application. Developers can evaluate security as they update and create pages.
I absolutely love this tool and it is a must for mitigating the security risks in your Apex application.
Apex Nitro : I've Never Worked So Efficiently As An Apex Developer

Apex Nitro is a really incredible tool and best of all it totally free.
Rather than writing javascript and CSS throughout your Apex application, Apex Nitro enables you to create centralised files that get uploaded to your application. Working in this way is highly efficient and saves you from trying to find code all over your application that may have to change.
As you make changes to the javascript or CSS files, you see the effects immediately in the browser without having to manually upload files or refreshing the page.
Using Apex Nitro also ensures that all of your developers are using the same files to ensure consistency throughout the application.
Also what is really cool is that any device pointed to the same server URL will be synchronised such that if you change your field values on a page in one brouser, it will reflect in all other browsers and devices at the same time. You can use this to see how your application appears on various screen sizes and on different browsers. All the silly issues with IE can be sorted while you do your development since you can run the application in multiple browsers at the same time.