  Tethering methods allow us to perform Monte Carlo simulations in ensembles
with conserved quantities. Specifically, one couples a reservoir to the
physical magnitude of interest, and studies the statistical ensemble where the
total magnitude (system+reservoir) is conserved. The reservoir is actually
integrated out, which leaves us with a fluctuation-dissipation formalism that
allows us to recover the appropriate Helmholtz effective potential with great
accuracy. These methods are demonstrating a remarkable flexibility. In fact, we
illustrate two very different applications: hard spheres crystallization and
the phase transition of the diluted antiferromagnet in a field (the physical
realization of the random field Ising model). The tethered approach holds the
promise to transform cartoon drawings of corrugated free-energy landscapes into
real computations. Besides, it reduces the algorithmic dynamic slowing-down,
probably because the conservation law holds non-locally.
