Ubuntu for Phones & Firefox OS both are latest Free and Open Source OS for mobile devices announced recently.
Ubuntu for Phones
To develop Ubuntu apps you just need to install Qt 5 and Ubuntu QML toolkit preview
1. Install Qt 5
sudo add-apt-repository ppa:canonical-qt5-edgers/qt5-beta1 && sudo apt-get update && sudo apt-get install qt5-meta-full && echo 'export PATH=/opt/qt5/bin:$PATH' >> ~/.bashrc
2. Install Ubuntu QML toolkit preview
sudo add-apt-repository ppa:ui-toolkit/ppa && sudo apt-get update && sudo apt-get install qt-components-ubuntu qt-components-ubuntu-demos qt-components-ubuntu-examples qt-components-ubuntu-doc notepad-qml
Then develop your fisrt Application.
Follow these steps.
Firefox OS also a powerful OS that enables the Open Web as a platform for mobile devices. Main feature of this OS is Consumer freedom means consumers who use devices powered by Firefox OS won’t be locked into one specific platform giving them more choice, flexibility and freedom. With Firefox OS, the Web is the platform. Developers can create amazing experiences and apps using HTML5 and the new Mozilla-proposed standard APIs. You can try Firefox OS simulator with your firefox browser.