I reckon I could confidently and authoritatively answer questions on:
[ul]
[li]Some web technologies (HTML, CSS, PHP, SQL)[/li][li]Card counting[/li][li]Poker[/li][li]Comparative religion (Mainly Christianity and Islam)[/li][li]Australian Rules Football[/li][/ul]
I’m an expert in many aspects of Windows system administration, particularly automating common tasks with batch scripting and VBScript.
If you need to identify all the workstations in your domain, search them for some files, move them into specific folders based on the date they were last modified, write the output to a comma-delimited text file, and email it to you, all without using any third-party software, then I’m your guy.
Libraries and librarianship, at least, and related topics. At one point I’d have said book and paper conservation, but I’ve forgotten most of that. Dealing with old houses, to an extent.
IMAX (and other giant-screen) films and theaters, digital cinema, and digital 3D cinema, and, as a hobby, high performance driving.
Writing, editing, and proofreading.
I’m a rodeo clown. I provide expert, factual information in GQ threads on the subject of rodeo clowns.
I have worked on the installation, repair, service and troubleshooting of commercial fire alarm systems for 31 years
U.S. retail banking (checking, savings, money markets and credit cards) and the client-side systems and procedures used for account maintenance. This accounts for most of the questions I’ve answered in GQ.
SCORM development and integration. Don’t worry, no one else knows what it is either.
Oh, and if for some reason you want to know anything about Tears For Fears, Radiohead, or Mesh, I’m your man as well. 
The US Federal Acquisition Regulations.
Don’t all jump at once, now. . . LOL!
An odd list: anatomy, botany, general biological sciences, watches, gardening (especially with cacti).
Biology, especially Biological Oceanography, with a particular emphasis on diatoms and vertical flux
Radiology, especially cardiothoracic.
Yup, that is an odd combo. When House scripts a case of hypersensitivity pneumonitis secondary to diatom exposure, I expect to be called.
Kites
Kiting
Kitemaking
Kiteflying
and occasionally…
Kite Crashing…(Back to the drawing board and all that)
And otherwise hob-nobbing with my fellow Kite-people!!
I like other stuff too, but heck, this is a chance to talk about kites!!!
Not much 
I’m always impressed and amazed that there are experts in just about anything here on the Dope.
NB
Organometallic chemistry
Risk management
Well, I’m not a freakin’ PhD, but by Doper standards, I’m an expert on the Israeli/Palestinian conflict and the Balkans. I rarely comment on the topic in which I’m earning a masters degree (public policy) because I don’t feel like getting involved in wankfests on how the government should operate. Besides, I usually get enough of that at school.
I typically find myself weighing in on threads covering:
**Guitar **- mostly rock and blues, a wide net covering players, music, gear, history, both electric and acoustic. I don’t have **picker’s **technique, but have immersed myself in guitar geekery for 30+ years now. And since I have spent a LOT of time figuring out what I like in a guitar and what various models have to offer - as well as built/assembled a couple of guitars - I have some solid perspective on what works and what doesn’t guitar-wise…if I do say so myself.
**Music **- in general, I have spent a lot of time schooling myself in music, radiating out from blues and rock, but inclusive of jazz, modern, classical, folk and a number of other genres.
**Books and Reading **- Not as active as I was, but I built a solid collection of First Edition books, and spent a lot of time part of that book-collecting scene. This is based on being a voracious reader across a variety of genres, and I enjoy discussing books online.
**Business Management and Strategic Planning **- I am part of the leadership team of a small company. In my role I am in charge of some operations and staff functions as well as head of Strategic Planning, having spents years learning the craft as a strategy consultant.
I have other stuff I am passionate about - I actively read about philosophy and science - but tend not discuss it as much online…
Oh, yeah. Since I’m a newsletter editor and publisher in the field of giant-screen theaters, I suppose I should have included these, and journalism, and everything else related to publishing a low-circulation print newsletter, too. I haven’t had formal training in any of these areas, just 12+ years of actually doing them.
Huh. I always thought you were a historian (as in, that was your job). You certainly know more about Asian history than anyone else here.
Counting stuff where I’ve got “actual” qualifications, I’m theoretically an expert on life insurance and variable annuities- I have held Series 7 and 66 licenses, and a Florida life, health and variable annuities brokers’ license, though all three have expired since I never actually worked in the field.
I have a degree in comparative politics and international relations, which makes me qualified to answer questions about international NGOs, Middle Eastern governments and international and US terrorist groups, and very little else. I think the majority of my factual contributions here that I didn’t Google have been in relation to US extremist groups, particularly white supremacist groups.
I’m an expert on sunglasses, particularly Oakley models from 1997-2006 (how’s that for uber-specialized?) as I’ve worked in retail and marketing in the field. I was also a fairly heavy contributor to O-review.com for a while, a fanzine/database so complete that Oakley staff use it to keep track of older designs and SKUs.
I’m reasonably qualified to be considered an “expert” on college fraternity operations and issues; I held twelve different executive/advisory board positions in my own chapter as an undergrad, and I’ve been an alumni advisor to the chapter and the international organization as a whole.
I work as a paralegal for a firm specializing in Florida workers’ compensation suits, so I’m reasonably qualified to advise people about workers’ compensation issues.
Getting into the “stuff I just know a lot about”, there’s Porsche history, Orlando theme parks and attractions (several of which I worked at), Nintendo history from 1992-2000, heavy metal and hard rock music from 1980-1992, and the auto industry from 1988-1996.
Geography and foreign political interaction - The amount I know is inversely proportional to it’s value in everyday life.
Officially I’ve been an expert witness on power system resource planning, fuel costing, and power market pricing in public proceedings for power plant hearings and rate case hearings.
After too many hours of siting on the witness stand playing mind games with lawyers I’m happy to say these are skills I no longer use.