  We describe neutron scattering, NMR relaxation, and thermal transport
properties of Z_2 spin liquids in two dimensions. Comparison to recent
experiments on the spin S=1/2 triangular lattice antiferromagnet in
kappa-(ET)2Cu2(CN)3 shows that this compound may realize a Z_2 spin liquid. We
argue that the topological `vison' excitations dominate thermal transport, and
that recent thermal conductivity experiments by M. Yamashita et al. have
observed the vison gap.
