The search for broken time-reversal symmetry (time-reversal symmetry breaking (TRSB)) in unconventional superconductors has intensified in the past few years as more systems have been predicted to possess such a state. Following our pioneering study of TRSB states in Sr2RuO4 using magneto-optic probes, we embarked on a systematic study of several other of these candidate systems. The primary instrument for our studies is the Sagnac magneto-optic interferometer that we recently developed. This instrument can measure magneto-optic Faraday or Kerr effects with an unprecedented sensitivity of 10 nrad at temperatures as low as 100 mK. In this paper, we review our recent studies of TRSB in several systems, emphasizing the study of the pseudogap state of high temperature superconductors and the inverse proximity effect in superconductor/ferromagnet proximity structures.