  We present the spectral and temporal properties of 15 candidate transient
X-ray sources detected in archival {\it XMM-Newton} and {\it Chandra}
observations of the nearby Magellanic-type, SB(s)m galaxy NGC 55. On the basis
of an X-ray color classification scheme, the majority of the sources may be
identified as X-ray binaries (XRBs), and six sources are soft, including a
likely supernova remnant. We perform a detailed spectral and variability
analysis of the data for two bright candidate XRBs. Both sources displayed
strong short-term X-ray variability, and their X-ray spectra and hardness
ratios are consistent with those of XRBs. These results, combined with their
high X-ray luminosities ($\sim 10^{38}~\rm erg~s^{-1}$), strongly suggest that
they are black hole (BH) binaries. Seven less luminous sources have spectral
properties consistent with those of neutron star or BH XRBs in both normal and
high-rate accretion modes, but one of them is the likely counterpart to a
background galaxy (because of positional coincidence). From the spectral
analysis, we find that the six soft sources are candidate super soft sources
(SSSs), with dominant emission in the soft (0.3--2 keV) X-ray band. Archival
{\it Hubble Space Telescope} optical images for 7 sources are available, and
the data suggest that most of them are likely to be high-mass XRBs. Our
analysis has revealed the heterogeneous nature of the transient population in
NGC 55 (6 high-mass XRBs, 1 low-mass XRBs, 6 SSSs, 1 active galactic nucleus),
helping establish the similarity of the X-ray properties of this galaxy to
those of other Magellanic-type galaxies.
