Fast Radio Bursts (FRBs) are enigmatic, millisecond-duration extragalactic transients discovered only in the late 2000s. Though the exact nature of their origin is unknown, their extremely short-duration emission endows several unique qualities that make them uniquely useful as probes of foreground matter. Chief among such properties is their dispersion. As FRB light propagates through plasma, their radio frequencies are dispersed, resulting in higher frequencies arriving before lower frequencies at Earth. The dispersion is directly proportional to the line-of-sight integral of the electron density weighted by the cosmological scale factor, i.e., the Dispersion Measure (DM). The DM of each FRB can be measured extremely precisely (~1%) by radio telescopes during detection. Thus, FRB DMs precisely probe foreground ionized matter, especially the warm, hot intergalactic medium (WHIM), a previously difficult-to-detect phase of matter. In this manuscript, I first describe the detection of the so-called "missing" baryons, a long-standing cosmological conundrum with FRBs. Then, I present work I have led that establishes the technique of foreground mapping, i.e., leveraging optical observations of foreground galaxies to constrain DM contributions from intervening foreground structures such as halos and cosmic-web filaments. Finally, I present the results of the FLIMFLAM survey, a statistical treatment of the foreground observation of several FRB sightlines to produce novel constraints on gas fractions within halos and filaments. I conclude with prospects for a FLIMFLAM-like analysis with a significantly larger sample of FRB sightlines expected to be detected within the next three years.