  We examined X-ray spectral and timing properties of ultraluminous X-ray
sources (ULXs) in nearby galaxies in XMM-Newton archival data. There appear to
be three distinct classes of spectra. One class shows emission from hot,
diffuse plasma. This thermal emission is similar to that seen from recent
supernovae; the temperatures are in the range 0.6-0.8 keV and the luminosities
are the lowest in our sample, near 10^39 erg/s. Three sources have spectra
which are strongly curved at high energies and have the highest temperatures in
our sample, 1.0-1.4 keV. These spectra are well fitted with a power-law plus
multicolor disk blackbody model with the power-law dominant at low energies or
a Comptonization model. The remainder of the sources are best fitted with a
power-law plus multicolor disk blackbody model, as is commonly used to describe
the spectra of accreting black holes. These sources have the lowest thermal
component temperatures, 0.1-0.4 keV, and extend to the highest luminosities,
above 10^40 erg/s. The temperature of the thermal component is in three
distinct ranges for the three source classes. This diversity of spectral shapes
and the fact that the sources lie in three distinct temperature ranges suggests
that the ULXs are a diverse population. Two ULXs which show state transitions
stay within a single class over the course of the transition. However, we
cannot conclude with certainty that the classes represent distinct types of
objects rather than spectral states of a single population of objects. We also
searched for timing noise from the sources and report detection of noise above
the Poisson level from five sources. In three of the sources, the power density
spectrum increases with decreasing frequency as a power-law down to the lowest
frequencies observed, below 10^-4 Hz.
