  The mass function of clusters of galaxies is determined from 400 deg^2 of
early commissioning imaging data of the Sloan Digital Sky Survey; ~300 clusters
in the redshift range z = 0.1 - 0.2 are used. Clusters are selected using two
independent selection methods: a Matched Filter and a red-sequence color
magnitude technique. The two methods yield consistent results. The cluster mass
function is compared with large-scale cosmological simulations. We find a
best-fit cluster normalization relation of sigma_8*omega_m^0.6 = 0.33 +- 0.03
(for 0.1 ~< omega_m ~< 0.4), or equivalently sigma_8 = (0.16/omega_m)^0.6. The
amplitude of this relation is significantly lower than the previous canonical
value, implying that either omega_m is lower than previously expected (omega_m
= 0.16 if sigma_8 = 1) or sigma_8 is lower than expected (sigma_8 = 0.7 if
omega_m = 0.3). The best-fit mass function parameters are omega_m = 0.19
(+0.08,-0.07) and sigma_8 = 0.9 (+0.3,-0.2). High values of omega_m (>= 0.4)
and low sigma_8 (=< 0.6) are excluded at >~ 2 sigma.
