GEOS

GRAPHIC ENVIRONMENT OPERATING SYSTEM is some very fine OO OS for various kinds of computers (including old 8-bit), that comes with a GUI and various office applications. One of its most notable aspects is that it isn't a resource pig: it already provided a GUI to 8-bit computers (Commodore 64/128, Apple ][), and multitasks fine even on 8088 computers (but takes advantage of Intel 32-bit mode when available). It has consequently been ported successfully to various hand-held platforms.