digital visual effects.
definitely not within the top of the field, but i think it fits within the 99.9% of the populace criterion.
also to a lesser extent military history and especially special operations history (although one hits a wall there when one isn’t in the military ;))
Optimality Theory, Northern California projectile points, *Olivella * shell beads, and Haliotis ornaments.
I’m not the world’s foremost authority on any but definitely good enough to get into the 0.1% club.
Wow…I nearly parallel you in areas 3, 4(somewhat) and 6. No wonder we keep winding up in the same threads. 
Ride a mean unicycle.
Bit of a Photoshop wiz.
Make good photos, too.
And a mildly disturbing talent for off the cuff poetry.
U.S. Constitutional law and federal jurisdiction.
Dunno if I’m in the top 0.1% of anything involving it. I suppose out of everybody on the planet I am.
Male homosexuality from a historical, sociological and anthroplogical aspect. o.O;
I don’t know anything about modern, American gayness though.
On a similar note - Yaoi. Which is Japanese comics for girls that depict romantic/sexual relationship between males.
Infectious disease. Have worked in Inection Control for past 15 years.
As a hobby I sequence MIDI music files.
i’m pretty much a jack of all trades, but professionally i know quite a bit about:
non-linear editing
photography
pro-sumer video
image manipulation (photoshop)
graphic/web design
fun stuff:
super puzzle fighter
tetris
indie music
CDRs
I’d say I’m in the top 1% when it comes to internal medicine and music of the late 20th century.
Top 0.1%? The music of Phish, and card magic. I used to be up there on ADHD, but I’ve fallen behind the curve in the last few years.
Dr. J
[Rezzing since it saves me the trouble of typing out a title and intro for a new topic]
Inspiring post by Exapno_Mapcase. He intimated that having 10 such areas would be exceptional.
Feel free to use the 99.9% threshold tho that isn’t a hard and fast rule. If you wish, you can put in your own % for each area. Your professional skills would presumably be your starting point.
Me:
Zoology specifically, biology generally (definitely more of a macro than micro type tho)
Mathematics, thru pre-algebra
Teaching skills
English grammar and writing skills (my sister recently expressed amazement at same, even tho they were just legal emails)
Reading comprehension in general
Auto racing, both history and how-to, tho I’ve never actually taken the wheel except for one track day in my own car.
WW2 history
Astronomy tho not at a pro level
Home electrical repairs and troubleshooting, but that’s what I was trained for long ago. Most people who are good handymen won’t touch electrical.
Leading people to a common goal. Harder than one might think sometimes.
Also, I am gud speler.
What about cooking? I thought you had culinary skills.
Oh hell, anybody can cook. It’s the gourmet folks who rise to heights that can be considered truly exceptional.
Most of you probably already know my answer: mathematics. Easily at the 99.9% level, possibly 99.99%.
My areas of expertise include cytopathology, raising figs and growing subtropical/tropical plants in northern climates.
Even though I easily know more about those subjects than 99.99% of the population, the other 0.01 or whatever percentage includes plenty of people whose knowledge trumps mine.
I’m so humble I can barely stand it.
99.9% of the population leaves out 8 million people. There are number of things I do as well or better than the 8 million others on earth, or at least did when I was younger. It isn’t easy with popular activities, but there are a lot of more narrow focused activities that might not even have 8 million other participants.
Then there are some super specific minor things that wouldn’t even have 8 million people that know anything about it, but those really feel like cheats. The PRMS bit above probably qualifies as one, but that kept me is good paying jobs for most of my career.
GIS/Mapping/Spatial Analysis. I’ve been in GIS for 34 years. When I started the acronym GIS wasn’t even yet used. It was called AM/FM. Automated Mapping/Facilities Mapping. At the first place I worked at I saw AM/FM awards and thought that that was odd combination. Mapping and radio 
I really don’t do the cartography much anymore. Well a little online stuff. Mostly I mash together data to get the data we need for applications, and automate that side of things. I now try to leave the UI to others.
Current title is GIS Applications Engineer. I think there may be one or two GIS people here on the SDMB, but can’t remember their names. Say hello if you see this.
Judging from my book sales, my area expertise is of absolutely no interest or relevance to 'way more than 99.99% of the population. Of Canada. Let alone the world.
Oh, dear, when this thread was started it was only 6 million people. It is getting harder and harder…
My area of expertise par excellence is my job: interpreting, mostly simultaneous, but I am even better at consecutive. There are not many who can claim to be professionals worldwide, not with my languge combination, and not with Spanish as A language (the language that is interpreted into). Say 500 are in my league, 1,000 if I am generous. But being in the 0.1% of the general population in your job does not feel like an exceptional achievement.
Last century’s Belgian comics, particularly Franquin’s (Gaston, Marsupilami, Spirou et Fantasio… that kind of stuff), and the Fabulous Furry Freak Brothers (and Fat Freddy’s Cat).
Probably typography. It’s obscure enough a subject. Not a professional by a long stretch, but have been dabbling in it for about 30 years, and still have not finished my own font (but getting closer).
Perhaps - only perhaps - Goya.