A cube six inches tall, with a circular glass door at the front and a little tray to put detergent in.
A washing machine only big enough to wash, at full load, a single pair of socks.
Easily fits into nooks and crannies. Never mind stairs, a cupboard under a single step would do. The hoses can be plugged into a nearby tap, but also come with adapters for the standardized thread of 2-litre bottles, so an inlet and outlet bottle can enable socks washing in remote, or on-the-go situations.
Relish in the zen-like process of pairwise socks cleansing. Contemplate the tribulations of life, two socks at a time. Ruminate on the possibility of finding an odd sock at the end of a wash.