The family of layered organic conductors, ƒÛ-(ET)2X, are model systems of interacting half-filled-band electrons on anisotropic triangular lattice. The bandwidth and the Coulomb repulsive energy are comparable in this family, which is situated around Mott transition. In particular, the Mott insulator, ƒÛ-(ET)2Cu2(CN)3, has nearly isotropic triangular-lattice network of transfer integral and therefore is a model system of frustrated quantum spins. At ambient pressure, there is no indication of magnetic ordering in either NMR spectra or relaxation rate, 1/T1, down to 30 mK. The spins are likely in the quantum liquid state. NMR, ƒÝSR and thermodynamic measurements suggest low-lying spin excitation at low temperatures with a signature of some kind of crossover or hidden order around 5K. This spin state is contrasting with that of the commensurate antiferromaget, ƒÛ-(ET)2Cu[N(CN)2]Cl, with more anisotropic triangular lattice. Under pressure, the aniferromagnet and the spin liquid both undergo Mott transition to the Fermi liquid, which shows superconductivity. The remarkable difference of the pressure-temperature phase diagrams of the spin liquid, ƒÛ-(ET)2Cu2(CN)3, and the anitiferromagnet, ƒÛ-(ET)2Cu[N(CN)2]Cl, indicates an interplay between the charge and spin degrees of freedom around the Mott transition. I discuss similarities and dissimilarities in the superconductivity-related properties of the two systems, highlighting the role of the spin frustration in the emergence of superconductivity. I also mention the doped triangular-lattice systems, ƒÛ-(ET)4Hg3-ƒÔX8 [X=Br, Cl]