Solidus originally comes from the open source project «Spree». The developer community disagreed about the future direction of Spree, for then a fork was created in 2015. From then on, both Spree and Solidus existed (read the whole story here).
Solidus is a free open source e-commerce platform that gives you full control over your shop. Solidus was built to support large volume businesses with different and complex requirements. Various use cases or special functions have already been integrated into the platform. This includes product management, inventory or taxonomy management. The frontend is fully customizable, as Solidus focuses much more on the backend (compared to Spree). Order management and shipping, multi-client capability with different countries, tax rates or currencies are all available. Nice, isn't it?