The key words are "not for hire." For a part 91 aircraft (i.e. most of
us here) operating not for hire (i.e. not carrying passengers or cargo
for pay) there is very little TSO requirement. I know the transponder
must be TSO'd (although the transponder itself is not absolutely
required). I was thinking that there is another piece of avionics that
requires a TSO, but if there is, I can't recall it at the moment. All
the regular stuff (radios, headsets, etc.) do not.

When you get into aircraft operating "for hire" (which does NOT include
the C-172 you rent at the local FBO) then TSO is required for a lot of
the equipment. Otherwise the ONLY advantage to a TSO approval is that
it is automatically "data acceptable to the administrator" and may make
the STC a bit easier to get if you are putting something unusual into
your plane.