  We obtain the harmonic measure of the hulls of critical percolation clusters
and Ising-model Fortuin-Kastelyn clusters using a biased random-walk sampling
technique which allows us to measure probabilities as small as 10^{-300}. We
find the multifractal D(q) spectrum including regions of small and negative q.
Our results for external hulls agree with Duplantier's theoretical predictions
for D(q) and his exponent -23/24 for the harmonic measure probability
distribution. For the complete hull, we find the probability decays with an
exponent of -1 for both systems.
