  We report on the first 180 days of RXTE observations of the outburst of the
black hole candidate IGR J17091-3624. This source exhibits a broad variety of
complex light curve patterns including periods of strong flares alternating
with quiet intervals. Similar patterns in the X-ray light curves have been seen
in the (up to now) unique black hole system GRS 1915+105. In the context of the
variability classes defined by Belloni et al. (2000) for GRS 1915+105, we find
that IGR J17091-3624 shows the \nu, \rho, \alpha, \lambda, \beta and \mu
classes as well as quiet periods which resemble the \chi class, all occurring
at 2-60 keV count rate levels which can be 10-50 times lower than observed in
GRS 1915+105. The so-called \rho class "heartbeats" occur as fast as every few
seconds and as slow as ~100 seconds, tracing a loop in the hardness-intensity
diagram which resembles that previously seen in GRS 1915+105. However, while
GRS 1915+105 traverses this loop clockwise, IGR J17091-3624 does so in the
opposite sense. We briefly discuss our findings in the context of the models
proposed for GRS 1915+105 and find that either all models requiring near
Eddington luminosities for GRS 1915+105-like variability fail, or IGR
J17091-3624 lies at a distance well in excess of 20 kpc or, it harbors one of
the least massive black holes known (< 3 M_sun).
