I can answer any question about fishing in So Cal. And a lot of questions about fishing in general.
Meteorology and MMORPG’s mainly. Other scattered stuff.
Chess.
Cold reading
Mentalism and magic in general
Pseudoscience and paranormal claims, and how to assess them
Thinking errors and how they can be reduced, if not eliminated
Technical, commercial and creative writing
Marketing for small businesses
How not to be very good at playing the guitar after 30 years of effort
What questions do I tend to answer?
Medicine questions, being a primary care doctor for over 25 years.
Prison questions, having worked in a prison for over 7 years.
Tolkien questions, having been a JRRT buff for over 35 years.
Addiction/alcoholism questions, having studied that specialty, and personally being in recovery for nearly 20 years.
I have multiple other small “islands of interest/expertise” too tiny to trumpet, but which I trot out when the occasion arises.
hmm. Telecommunications, especially construction of new infrastructure. Fiber optic runs, towers, all that stuff.
Beer and cooking. Grilling and beer drinking especially.
Military history, but I’m a rank amateur compared to many around here.
History-type stuff and sometimes critters of various sorts, plus tangents spinning off the two.
Well, crap. Then in my case some training, not so much the professional authority :p. I don’t know that I’ve ever answered a technical question relating to my actual job ( which somewhat bores me and I’m not an expert on, anyway ).
Personally I’m far more the barely ranked amateur in the areas above than the professional and I hope I’ve never pretended otherwise. My tendency towards pedantry comes from the same impulse that made me into a professional-student-as-hobby for 13 years. I usually try to have some sort of factual citation accessible if need be. But I also at least make an attempt at being humble when I fuck up, as I defintely do from time to time ;).
I don’t really have a problem with amateurs answering GQs ( I’d be a horrible hypocrite if I did ), nor do I think particular professionals are invariably more correct. But certainly some credentials are to be preferred, all else being equal.
I will answer questions about flooding, the National Flood Insurance Program, stormwater, rainfall/runoff, dams, and other hydrology & hydraulics issues.
I’m one of the biology squad, being a grad student in molecular biology. There are a few of us who can answer most questions pretty authoritatively, and there are certainly plenty of members who know more than I do. There are a lot more that think they do…
Special effects for computer games. Yeah, it’s pretty specific.
Areas of expertise (as opposed to being an “expert”): medicine, particularly pathology and laboratory medicine and ornamental gardening.
I don’t consider myself an expert because I don’t have my PhD yet, but I can answer more questions about math, computer science and statistics than most people around here.
I know a lot about Shakespeare, but so do several other Dopers. I would venture to say that I probably do know more about non-Shakespearean early modern drama, and about popular Elizabethan literature in general, than anyone else on these boards. Alas, questions about Thomas Deloney almost never come up in GQ 
I find the implication that there may be things in which I am not expert offensive.
Canadian appellate practise.
Win XP and the perspective necessary to get through the really tough days.
As a street-level activist with many years of experience, I can speak knowledgably about the theory and history of anarchism, labour organizing principles, and practical law, both civil and criminal.
I have a passing familiarity with post-modern, existential, and phenomenological philosophy, with a particular interest in Kant and Schopenhauer. Through existentialism and the alienists, I have an interest in Freudian and Jungian psychology, as well as speculative anthropology, particularly as it pertains to the tradition of mysticism.
Full Fee Paying International Students (FFPIS) and international exchange regulations. Also, government funding for Australian schools.
Architecture as I have been a practicing Architect for 30 years. I will also answer a fair amount of construction related questions, but consider a General Contractor the expert in that arena, although I do know a fair amount about it.
Retirement savings–not an expert but I feel I do pretty damn good at it as I am on target to retire in 5 years at age 55. If nothing else I can direct people to some good websites that will help them educate themselves on the issues involved and to get them motivated to do it and not just talk about it. Based on what I have read here I am likely one of the wealthier posters here at least relative to net worth.
Exercise and diet–again not an expert but at age 50 I am in the top 10% for my age group as I take very good care of myself and exercise and watch my diet. So I feel I can contribute with some degree of knowledge to those questions, but of course would yield to a nutritionist or a trainer as to expertise.
Children–not an expert–but then who is? But I can give my perspective as a father to a teenager!
I don’t think I’m comfortable saying “expert”, but I usually try to contribute to a thread about medical lab tests, blood donation, that sort of thing. I’ve only been a medical technologist for a few years now, but I like to think that I’m sometimes helpful answering questions about it.