  I suggest that there are two classes of ultraluminous X-ray sources (ULXs),
corresponding to super-Eddington mass inflow in two situations: (a)
thermal-timescale mass transfer in high-mass X-ray binaries, and (b)
long-lasting transient outbursts in low-mass X-ray binaries. These two classes
are exemplified by SS433 and microquasars like GRS 1915+105 respectively. The
observed ULX population is a varying mixture of the two, depending on the star
formation history of the host galaxy. ULXs in galaxies with vigorous star
formation (such as the Antennae) are generally SS433--like, while ULXs in
elliptical galaxies must be of the microquasar type. The latter probably have
significantly anisotropic radiation patterns. They should also be variable, but
demonstrating this may require observations over decades. The close analogy
between models of X-ray binaries and active galactic nuclei (AGN) suggests that
there should exist an apparently super-Eddington class of the latter, which may
be the ultrasoft AGN, and a set of X-ray binaries with Doppler--boosted X-ray
emission. These are presumably a subset of the ULXs, but remain as yet
unidentified.
