For myself, that is probably primarily military topics. As in, that has been my career for over 20 years. First enlisting when President Reagan was in his first term, and still serving today under my seventh President. In two branches of service, four countries, and on bases of all 4 branches of service. Infantry, Missile Defense, and Computers were my main areas of specialization there, but also security, weapons and urban combat techniques, supply, logistics, as well as working in field hospitals.
And as a large believer in what George Santayana said about learning from history. As such, I am an expert in WWII history, but also well researched on other things from the Neolithic to late 20th century. Human Migrations and how one civilization forcing another to move has been a particular interest there. I believe in the axiom that what has happened before will happen again, and try to spot trends in past events to see what may happen in the future.
Computers is another one, comes from having a mother that was a programmer in the 1960's and growing up with the things. Still very active in this field, but the demand has largely died as people ditch computers for other devices. Finally closed my store 3 years ago when all people brought in for me to fix were cheap netbooks that were not even worth fixing in the first place.
And a bit more esoteric, being a DJ. Started that professionally in 1981, everything from skating rinks and teen dance clubs to more modern dance clubs and even strip clubs in five states and two other countries. Want to know what it was like trying to use a guitar mixer before the "modern DJ mixer" came out, or the best way to match tempo and do the old-school "DJ Music Wave" along with transitions between genres, feel free to ask. I no longer do it as hearing damage (permanent tinnitus) prevents me from perusing this professionally anymore, but I still keep my finger on the equipment and trends from personal interest.
And as a side into that, broadcasting. Mostly at a small defunct UHF TV station in the LA area, but doing it for many years (mostly as a show lead, researcher, and equipment operator). One thing about working at a small TV station, not many people there so everybody gets to do almost anything. "Hey, there is a 30 minute hole in the schedule, wanna do a show?" "Hey, we got nothing scheduled from midnight to 2 in the morning. Go drag one of those public domain horror movies out of the archive, you got an hour to make and film the skits and sketches to open and close it with."
What kind of expertise do others in here have, and would be interested in sharing? Either for helping background in writing stories, or just to learn more about different things.