I'm going to give a few examples to add to the other answers.
Spin liquids are low-temperature magnetic phases of matter which do not spontaneously break any symmetries. Generally, some type of frustration prevents the system from adopting any particular ground state, the origin of frustration could be competing energetic interactions or can be caused by geometric arrangements of the magnetic ions which prevent any ground state from being selected, and thus the system avoids ordering, remaining "liquid like". There are both classical and quantum versions of spin liquids. Classically, they are characterized by a macroscopically large set of ground states, such as the Kagome and pyrochlore antiferromagnets, and correspondingly a non-zero entropy at very low temperatures. These systems are interesting because they exhibit interesting emergent properties: in particular, the dipolar pyrochlore magnets Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ exhibit emergent magnetic monopole excitations. In the quantum case, one can have a massive superposition of the degenerate classical groundstates, similar to Anderson's Resonating Valence Bond (RVB) liquid model, originally proposed to explain some of the properties of high-temperature cuprate superconductors, and these superpositions generally lead to a large amount of entanglement, meaning that the ground state is not a product state. These quantum spin liquids can have all sorts of interesting properties, and on the pyrochlore lattice can exhibit emergent Quantum Electrodynamics (QED), including an emergent photon excitation. The pyrochlore spin liquid (called quantum spin ice for reasons I won't explain here) is an example of a gapless spin liquid: the photon excitation is gapless, meaning it requires only an infinitesimal amount of energy to excite the system. Much more common are gapped spin liquids, which are easier to understand: since they are gapped, at low temperatures the ground state will be stable and excitations will be exponentially suppressed. It is then possible (using the methods originally developed to my knowledge by Xiao-Gang Wen) to integrate out the excitations and obtain a gauge theoretic model of the low-energy spin liquid phase, which can include many interesting topological properties. A famous example is the Kitaev spin liquid, which has anyonic excitations.
The key to spin liquid physics is that the ground state is highly entangled and does not break any symmetry, in contrast to systems like ferromagnets whose ground states are symmetry broken states. Understanding the nature of the ground state wavefunction allows one to understand the low-lying excitation spectrum and describe the low-temperature physics.
One can also study quantum phase transitions: zero-temperature phase transitions which occur as an external variable such as magnetic field or pressure are varied. These are entirely described in terms of the change in the ground state of the system at some critical value of the external control parameter, a simple example being the transverse field Ising model.
Symmetry broken groundstates are also of interest and are plentiful and easy to find. The simplest example probably is the ferromagnet, which has a global spin rotation symmetry which is spontaneously broken at the critical temperature as the system orders. The ground state of a ferromagnet is a simple product state (all spins point in the same direction, i.e. an effectively classical state), which by itself is unremarkable, but one can still understand the low energy excitations (magnons) once one knows the ground state. Some more complicated quantum examples of symmetry broken phases are superfluids, (BCS) superconductors, and Bose-Einstein Condensates (BEC's). Both superfluids and BCS superconductors can be thought of as BEC's in a way, in that they are "adiabatically connected to" a BEC state, i.e. I can "continuously deform" the ground state wavefunction to reach a BEC wavefunction. Again, for the purposes of understanding the low-energy physics, understanding the ground state is crucial, as it contains much of the interesting useful information for understand the low-energy properties of these systems.