  We report the discovery of a supernova (SN) with the highest apparent energy
output to date and conclude that it represents an extreme example of the Type
IIn subclass. The SN, which was discovered behind the Large Magellanic Cloud at
z = 0.289 by the SuperMACHO microlensing survey, peaked at M_R = -21.5 mag and
only declined by 2.9 mag over 4.7 years after the peak. Over this period, SN
2003ma had an integrated bolometric luminosity of 4 x 10^51 ergs, more than any
other SN to date. The radiated energy is close to the limit allowed by
conventional core-collapse explosions. Optical spectra reveal that SN 2003ma
has persistent single-peaked intermediate-width hydrogen lines, a signature of
interaction between the SN and a dense circumstellar medium. The light curves
show further evidence for circumstellar interaction, including a long plateau
with a shape very similar to the classic SN IIn 1988Z -- however, SN 2003ma is
ten times more luminous at all epochs. The fast velocity measured for the
intermediate-width H_alpha component (~6000 km/s) points towards an extremely
energetic explosion (> 10^52 ergs), which imparts a faster blast-wave speed to
the post-shock material and a higher luminosity from the interaction than is
observed in typical SNe IIn. Mid-infrared observations of SN 2003ma suggest an
infrared light echo is produced by normal interstellar dust at a distance ~0.5
pc from the SN.
