A popular open-source testing tool for mobile applications is Appium, which automates tests for platforms like Android, iOS, and Windows. It is used to automate the testing of native, hybrid, and web applications. This tool is quite popular due to its availability, free nature, low memory consumption and integration with other external applications such as Selenium WebDriver and Selenium GRID. The tool is cross-platform, MEANING the same test can be run on multiple platforms. Appium can test multiple devices simultaneously. 

Appium also offers the following additional advantages to testers and developers: 

• Executes multiple test SCRIPTS simultaneously.
• Includes a tool for recording test cases and a playback tool for replaying those recordings.
• Allows testing various elements simultaneously (e.g., emulators, real devices, and simulators).
• Cloud-based testing is supported using Testdriod.
• Supported by an active community (GOOGLE group) that enables TROUBLESHOOTING to be done more quickly and easily.