Is there a listing anywhere of the "normal" mutual aid channels?
All depends on your City/County/State.
Here in Boston we have Mutual Aid Fire Districts and Police Associations that have many mutual aid frequencies for incidents [Bolo's, Fires etc]
Check out the RR WIKI's as well as the RR DB
I live in the Adirondacks of upstate NY, just south of Plattsburgh. I know there is quite a bit of Federal activity in the area with the border not far away. I know NYSP Car to Car 154.665 (PL 110.9) and 155.370 is used quite often, but I am unaware of anything else. I often here mention of police switching to IBET car to car but I am unsure of what it is.