  We analyze 18 sources that were found to show interesting properties of
periodicity, very soft spectra and/or large long-term variability in X-rays in
our project of classification of sources from the 2XMMi-DR3 catalog but were
poorly studied in the literature, in order to investigate their nature. Two
hard sources show X-ray periodicities of ~1.62 hr (2XMM J165334.4-414423) and
~2.1 hr (2XMM J133135.2-315541) and are probably magnetic cataclysmic
variables. The source 2XMM J123103.2+110648 is an active galactic nucleus (AGN)
candidate showing very soft X-ray spectra (kT~0.1 keV) and exhibiting an
intermittent ~3.8 hr quasi-periodic oscillation. There are six other very soft
sources (with kT<0.2 keV), which might be in other galaxies with luminosities
between ~10^{38}-10^{42} erg/s. They probably represent a diverse group that
might include objects such as ultrasoft AGNs and cool thermal disk emission
from accreting intermediate-mass black holes. Six highly variable sources with
harder spectra are probably in nearby galaxies with luminosities above 10^{37}
erg/s and thus are great candidates for extragalactic X-ray binaries. One of
them (2XMMi J004211.2+410429, in M 31) is probably a new-born persistent
source, having been X-ray bright and hard in 0.3--10 keV for at least four
years since it was discovered to enter an outburst in 2007. Three highly
variable hard sources appear at low galactic latitudes and have maximum
luminosities below ~10^{34} erg/s if they are in our Galaxy, thus great
candidates for cataclysmic variables or very faint X-ray transients harboring a
black hole or neutron star. Our interpretations of these sources can be tested
with future long-term X-ray monitoring and multi-wavelength observations.
