Android is certainly leading the race of open source Mobile OS. However,2013 will see launch of devices loaded with different OS.In Mobile World Congress 2013, Mobile operating systems like Firefox,Tizen,Sailfish,Ubuntu were presented in their respective booths.
FIREFOX OS :
Firefox OS is a new mobile operating system developed by Mozilla's Boot to Gecko (B2G) project. It uses a Linux kernel and boots into a Gecko-based runtime engine, which lets users run applications developed entirely using HTML, Javascript, and other open web application APIs.
Go to FIREFOX OS HOMEPAGE for more details.
TIZEN OS :
It is an open source system that aims to offer a consistent user experience across devices. Tizen's main components are the Linux Kernel and the Webkit runtime. The Tizen project resides within the Linux Foundation and is governed by a Technical Steering Group (TSG) composed of Intel and Samsung. source:Wikipedia
SAILFISH OS:
The Sailfish OS is a mobile-optimized operating system that has the flexibility, ubiquity and stability of the Linux core with a cutting edge user experience built with the renowned Qt™ platform.
UBUNTU MOBILE OS :
Ubuntu is the open source desktop Linux software adored by geeks all over.The Ubuntu Touch mobile OS is backed by a British company Canonical, it strives to be an elegant and immersive gesture-based experience.
HTML5 is fully supported but Ubuntu is not limited to HTML5. To use the full power of the hardware, you can develop native apps, using OpenGL and QML, with C or C++ and Javascript compiled for extra performance.Major game developers like EA, Valve and Unity Technologies are now committing to Ubuntu. source:Ubuntu