Those that I find the most useful are those that I (and likely many others) tend to take for granted.

For example, fuzzy logic may very well be used in electronics that involve temperature control - fridge, aircon, rice cooker, water heater - under the hood.

Another one is CSP (constraint-satisfaction problems) solvers which tend to be used in scheduling softwares. A possible use case is public transportation.

There are probably lots more AIs working behind the scenes that benefit everyone, but don't get the coverage because they are just boring tech now. People may not even consider them AI!

I appreciate these AI for making my life so convenient.