  We present the first simultaneous, multi-wavelength observations of an L
dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with
the Very Large Array, the Chandra X-ray Observatory, and the Kitt Peak 4-m
telescope. We detect strongly variable and periodic radio emission (P=3 hr)
with a fraction of about 60% circular polarization. No X-ray emission is
detected to a limit of L_X/L_{bol}<2e-5, several hundred times below the
saturation level observed in early M dwarfs. Similarly, we do not detect
H-alpha emission to a limit of L_{H-alpha}/L_{bol}<2e-7, the deepest for any L
dwarf observed to date. The ratio of radio to X-ray luminosity is at least four
orders of magnitude in excess of that observed in a wide range of active stars
(including M dwarfs) providing the first direct confirmation that late-M and L
dwarfs violate the radio/X-ray correlation. The radio emission is due to
gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which
is maintained on timescales longer than three years. The detected 3-hour period
may be due to (i) the orbital motion of a companion at a separation of about
five stellar radii, similar to the configuration of RS CVn systems, (ii) an
equatorial rotation velocity of about 37 km/s and an anchored, long-lived
magnetic field, or (iii) periodic release of magnetic stresses in the form of
weak flares. In the case of orbital motion, the magnetic activity may be
induced by the companion, possibly explaining the unusual pattern of activity
and the long-lived signal. We conclude that fully convective stars can maintain
a large-scale and stable magnetic field, but the lack of X-ray and H-alpha
emission indicates that the atmospheric conditions are markedly different than
in early-type stars and even M dwarfs. [abridged]
