  We report results of a deep all-sky search for periodic gravitational waves
from isolated neutron stars in data from the S6 LIGO science run. The search
was possible thanks to the computing power provided by the volunteers of the
Einstein@Home distributed computing project. We find no significant signal
candidate and set the most stringent upper limits to date on the amplitude of
gravitational wave signals from the target population. At the frequency of best
strain sensitivity, between $170.5$ and $171$ Hz we set a 90% confidence upper
limit of ${5.5}^{-25}$, while at the high end of our frequency range, around
505 Hz, we achieve upper limits $\simeq {10}^{-24}$. At $230$ Hz we can exclude
sources with ellipticities greater than $10^{-6}$ within 100 pc of Earth with
fiducial value of the principal moment of inertia of $10^{38} \textrm{kg m}^2$.
If we assume a higher (lower) gravitational wave spindown we constrain farther
(closer) objects to higher (lower) ellipticities.
