The modules for Handspring's Springboard Expansion Slot can be thought of as apps installed on a mobile operating system [COMPACTIDEA]

If you think about it broadly, there isn't much of a difference between the hardware modules that were available for Handspring's line of PDAs, and the software apps that we routinely install on mobile OSes such as Android or Windows Phone. The PDA device itself can be thought of as an OS, which provides a platform for expansion. And we insert various hardware modules into the PDA to add/provide specific capabilities. Sounds just like what applications do to an operating system.