  We demonstrate steady-state, mirrorless superradiance in a cold vapor pumped
by weak optical fields. Beyond a critical pump intensity of 1 mW/cm$^2$, the
vapor spontaneously transforms into a spatially self-organized state: a density
grating forms. Scattering of the pump beams off this grating generates a pair
of new, intense optical fields that act back on the vapor to enhance the atomic
organization. We map out experimentally the superradiant phase transition
boundary and show that it is well-described by our theoretical model. The
resulting superradiant emission is nearly coherent, persists for several
seconds, displays strong temporal correlations between the various modes, and
has a coherence time of several hundred $\mu$s. This system therefore has
applications in fundamental studies of many-body physics with long-range
interactions as well as all-optical and quantum information processing.
