When I started with Arduino the Main Site docs included the goal of making stand-alone MCU chips. People made in-field data collectors that ran 1 to 3 months on AA cells, fit in an Altoids tin.
ATmega and ATtiny chips need ground, power and 2 bypass caps to run if you repurpose the reset pin and use the internal clock source.
It would beed to be wired up to do anything yah, but even with a led13, reset button and crystal&caps there is not much build bulk required.