The article reports the development of a class of micro-architected materials that maintain a nearly constant stiffness per unit mass density, even at ultra-low densities. These materials, referred to as "mechanical metamaterials," are designed to have high structural connectivity and nanoscale features, with structural members capable of carrying loads in tension or compression. The production of these microlattices, which can be made from polymers, metals, and ceramics, is achieved using projection microstereolithography, an additive manufacturing technique combined with nanoscale coating and post-processing. The materials exhibit ultra-stiff properties across more than three orders of magnitude in density, regardless of the constituent material. The stretch-dominated unit cell structure, consisting of b struts and j frictionless joints, significantly improves mechanical efficiency compared to bend-dominated structures, leading to higher stiffness-to-weight ratios. The study also explores the effects of loading direction and lattice orientation on the $E-\rho$ scaling relationship, demonstrating that the stretch-dominated microlattices populate the highly desirable ultra-light, ultra-stiff space.The article reports the development of a class of micro-architected materials that maintain a nearly constant stiffness per unit mass density, even at ultra-low densities. These materials, referred to as "mechanical metamaterials," are designed to have high structural connectivity and nanoscale features, with structural members capable of carrying loads in tension or compression. The production of these microlattices, which can be made from polymers, metals, and ceramics, is achieved using projection microstereolithography, an additive manufacturing technique combined with nanoscale coating and post-processing. The materials exhibit ultra-stiff properties across more than three orders of magnitude in density, regardless of the constituent material. The stretch-dominated unit cell structure, consisting of b struts and j frictionless joints, significantly improves mechanical efficiency compared to bend-dominated structures, leading to higher stiffness-to-weight ratios. The study also explores the effects of loading direction and lattice orientation on the $E-\rho$ scaling relationship, demonstrating that the stretch-dominated microlattices populate the highly desirable ultra-light, ultra-stiff space.