Heat exchangers are a big one, but they are not yet that common in the US. They are used to pre-heat the makeup air. I'm looking into one atm as part of a HVAC upgrade as currently running a bath fan with all the windows closed will draw air down the chimney.
Heat exchangers have been code for more than 20 years. Maybe not common, but only because there are a lot of old houses. Or maybe where you live didn't adopt the new code?
I have a house that's less than 20 and it doesn't have a heat exchanger. I haven't heard of the new construction around here having any. Perhaps it's only in certain states?
