I’d say that smoke detectors and heat detectors are the most common with flame detectors and fire-gas detectors being less common.
[quote]
•Smoke detectors sense visible or invisible particles of combustion generated by burning, smoldering, or the incipient stage of combustion. These devices fall into two categories -- photoelectric and ionization.
•Thermal detectors sense the high temperature or the temperature rise caused by a fire.
•Flame detectors sense the radiation produced by a fire.
•Fire-gas detectors sense the gases produced by a fire.
•Other detectors sense some phenomenon other than smoke, thermal, flame, or fire-gas to detect a fire.
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Common for what you would see.
There are millions of specialty installations you will never see.
“unique” installs exist in the “non-public” side of property’s all over the country where extremely sensitive detection is required.
Paint factories, specialty laboratories, and government facility’s all fall on that list.
Think on a larger scale than what you could even imagine exists 
There is some pretty COOL stuff out there.
I have a brochure in my closet describing a smoke detection system that looks for flames and smoke particles in a security camera feed. A lot of places use it, apparently.