Why does the quark-gluon plasma at RHIC behave as a nearly ideal fluid?

This article gives a brief review in the following areas. (i) Collective flow phenomena in heavy ion collisions. The data from RHIC indicate robust collective flows, well described by hydrodynamics with the expected equation of state. The transport properties turned out to be unexpected, with very small viscosity. (ii) The physics of highly excited matter produced in heavy ion collisions at T_c<T<4T_c, different from that of weakly coupled quark-gluon plasma because of the relatively strong coupling generating bound states of quasiparticles. (iii) Wider discussion of other "strongly coupled systems", including strongly coupled supersymmetric theories studied via Maldacena duality, as well as recent progress as regards trapped atoms with very large scattering lengths.