The present disclosure provides a portable processing unit add-on computing device for mobile devices. In one embodiment, the portable processing unit add-on comprises one or more processors, memory and means for remotely displaying and receiving input for its user interface. The portable processing unit add-on interacts with the mobile device in order to allow the mobile device's user to access the portable processing unit's functionality. The portable processing unit typically, but not necessarily, includes a desktop-grade processor, such as one based on the Intel™ x86 architecture, and runs a desktop-grade operating system such as Microsoft Windows™ or Ubuntu Linux™.