  We present a framework for forecasting cosmological constraints from future
neutral hydrogen intensity mapping experiments at low to intermediate
redshifts. In the process, we establish a simple way of comparing such surveys
with optical galaxy redshift surveys. We explore a wide range of experimental
configurations and assess how well a number of cosmological observables (the
expansion rate, growth rate, and angular diameter distance) and parameters (the
densities of dark energy and dark matter, spatial curvature, the dark energy
equation of state, etc.) will be measured by an extensive roster of upcoming
experiments. A number of potential contaminants and systematic effects are also
studied in detail. The overall picture is encouraging -- if autocorrelation
calibration can be controlled to a sufficient level, Phase I of the SKA should
be able to constrain the dark energy equation of state about as well as a DETF
Stage IV galaxy redshift survey like Euclid, in roughly the same timeframe.
