MSW neutrino oscillations: new aspects of old solutions

We review some new or less known aspects of the MSW (Mikheyev-Smirnov-Wolfenstein) solutions to the solar neutrino problem. In particular, we discuss solutions involving (i) low mass square difference and nearly maximal mixing; (ii) three-flavor oscillations; (iii) νe mixing with both active and sterile neutrinos. Possible tests of such solutions are discussed, including seasonal variations related to the Earth regeneration effect, as well as a careful comparison of (heavy-)water Cherenkov experimental observables. (authors)