A collection of hydrogen atoms (not molecules!) in which all protons and electrons have the same spin. Spin polarized hydrogen is interesting because, neglecting longer time scale effects such as spin relaxation, the atoms cannot combine into a molecule of H_2. This is due to the Pauli Exclusion Principle - since a molecule of H_2 has both electrons sharing a two-atom orbital, they must have opposite spins. But that is impossible in a spin polarized gas, since the spins are all parallel. This behavior, besides being interesting in its own right, allows an awesome degree of cooling, even to the point of Bose-Einstein Condensation.