  We carry out highly accurate \emph{ab initio} path integral Monte Carlo
(PIMC) simulations to directly estimate the free energy of various warm dense
matter systems including the uniform electron gas and hydrogen without any
nodal restrictions or other approximations. Since our approach is based on an
effective ensemble in a bosonic configuration space, it does not increase the
computational complexity beyond the usual fermion sign problem. Its application
to inhomogeneous cases such as an electronic system in a fixed external ion
potential is straightforward and opens up the enticing possibility to benchmark
density functional theory and other existing methods. Finally, it is not
limited to warm dense matter, and can be applied to a gamut of other systems
such ultracold atoms and electrons in quantum dots.
