  We review recent theoretical and experimental developments concerning
collective spin excitations in two-dimensional electron liquid (2DEL) systems,
with particular emphasis on the interplay between many-body and spin-orbit
effects, as well as the intrinsic dissipation due to the spin-Coulomb drag.
Historically, the experimental realization of 2DELs in silicon inversion layers
in the 60s and 70s created unprecedented opportunities to probe subtle quantum
effects, culminating in the discovery of the quantum Hall effect. In the
following years, high quality 2DELs were obtained in doped quantum wells made
in typical semiconductors like GaAs or CdTe. These systems became important
test beds for quantum many-body effects due to Coulomb interaction, spin
dynamics, spin-orbit coupling, effects of applied magnetic fields, as well as
dissipation mechanisms. Here we focus on the recent results involving chiral
effects and intrinsic dissipation of collective spin modes: these are not only
of fundamental interest but also important towards demonstrating new concepts
in spintronics. Moreover, new realizations of 2DELs are emerging beyond
traditional semiconductors, for instance in multilayer graphene, oxide
interfaces, dichalcogenide monolayers, and many more. The concepts discussed in
this review will be relevant also for these emerging systems.
