As you have mentioned that you need Linux and OsX supported software/packages, here are some:
- Python's automac package: Automated Testing on Mac - test GUI applications
- Appium for mac: Proof of concept for automating a mac app with JSON wire protocol
- Sikuli Script: (Cross Platform) automates anything you see on the screen. It uses image recognition to identify and control GUI components. It is useful when there is no easy access to a GUI's internal or source code
- Autopy: A simple, cross-platform GUI automation toolkit for Python
In case, you want one for Windows, then AutoIT should be your choice.