  With magnetic fields below one microgauss, the minimum pressures of
synchrotron plasmas start to approach those in the thermal gas in cluster
outskirts and the more diffuse WHIM in large-scale structure filaments. We
summarize some of our techniques to find the corresponding low surface
brightness radio sources and what we have uncovered. We identify cluster-like
sources in poor environments, likely requiring very efficient relativistic
particle acceleration, as well as very diffuse radio galaxies with no
indications of current activity. More detailed observations of Coma and a new
radio-identified cluster highlight some emerging issues in cluster-related
shocks: Coma has a shock on the border of its halo, suggesting a connection
between the two; the Coma relic appears to be an "infall" shock, associated
with a 500 km/s infalling column of galaxies; the new cluster has filamentary
pieces in addition to its more classical halo and peripheral relics, suggesting
that we are beginning to see relic emission face on. Finally, a different kind
of shock comes from our recent finding of ambiguities in the determination of
rotation measures which will influence cluster magnetic field studies.
