  The families of organic charge-transfer salts $\kappa$-(BEDT-TTF)$_2X$ and
$\kappa$-(BETS)$_2X$ have proven to serve as a powerful playground for the
investigation of the physics of frustrated Mott insulators. These materials
have been ascribed model character, since dimerization of the organic molecules
allows to map these materials onto a single band Hubbard model, in which the
dimers reside on an anisotropic triangular lattice. By changing the inorganic
unit $X$ or applying physical pressure, the correlation strength and anisotropy
of the triangular lattice can be varied. This has lead to the discovery of a
variety of exotic phenomena, including quantum spin liquid states, a plethora
of long-range magnetic orders in proximity to a Mott metal-insulator
transition, and unconventional superconductivity. While many of these phenomena
can be described within this effective one-band Hubbard model on a triangular
lattice, it became evident in recent years that this simplified description is
insufficient to capture all observed magnetic and electronic properties. The
ingredients for generalized models that are relevant include, but are not
limited to, spin-orbit coupling, intra-dimer charge and spin degrees of
freedom, electron-lattice coupling, as well as disorder effects. Here, we
review selected theoretical and experimental discoveries that clearly
demonstrate the relevance thereof. At the same time, we outline that these
aspects are not only relevant to this class of organic charge-transfer salts,
but are also receiving increasing attention in other classes of inorganic
strongly correlated electron systems. This reinforces the model character that
the $\kappa$-phase organic charge-transfer salts have for understanding and
discovering novel phenomena in strongly correlated electron systems from a
theoretical and experimental point of view.
