  We study an unconventional quantum Hall effect for the surface states of
ultrathin Floquet topological insulators in a perpendicular magnetic field. The
resulting band structure is modified by photon dressing and the topological
property is governed by the low-energy dynamics of a single surface. An
exchange of symmetric and antisymmetric surface states occurs by reversing the
light's polarization. We find a novel quantum Hall state in which the zeroth
Landau level undergoes a phase transition from a trivial insulator state, with
Hall conductivity $\sigma_{yx}=0$ at zero Fermi energy, to a Hall insulator
state with $\sigma_{yx}=e^2/2h$. These findings open new possibilities for
experimentally realizing nontrivial quantum states and unusual quantum Hall
plateaux at $(\pm1/2,\pm3/2,\pm5/2,...)e^2/h$.
