  The recently discovered fast radio bursts (FRBs), presumably of
extra-galactic origin, have the potential to become a powerful probe of the
intergalactic medium (IGM). We point out a few such potential applications. We
provide expressions for the dispersion measure and rotation measure as a
function of redshift, and we discuss the sensitivity of these measures to the
HeII reionization and the IGM magnetic field. Finally we calculate the
microlensing effect from an isolate, extragalctic stellar-mass compact object
on the FRB spectrum. The time delays between the two lensing images will induce
constructive and destructive interference, leaving a specific imprint on the
spectra of FRBs. With a high all-sky rate, a large statistical sample of FRBs
is expected to make these applications feasible.
