Chess. I play in tournaments all the time and study intensively. If you count the $50 I get for teaching some kids per month, and the tournament cash I win (about $150 this year), I guess you could say I make money at it, though. About enough to buy a tank of gas…
Nietzsche. I know a lot about Friedrich Nietzsche. I’m more secure in my knowledge of Friedrich Nietzsche than I am in my knowledge of Spanish or immigration, which are basically the top two subjects in my career track. I don’t know only Nietzsche’s writings, I know about his childhood and the way he lived as an adult and how his experiences shaped his philosophy. Thanks to a professor of mine, I have a lot of creepy anecdotes to share about his personal life, which are even weirder than his philosophy.
I also know a great deal about mental health – way more than the average person. Whether this is something that will aid me in my future career is something that remains to be seen – though at present I have no formal training in this area, just a wealth of knowledge based on personal research, experience, and osmosis from my formally educated husband. Slightly related but not the same, I’m fascinated by social psychology and have read a lot of books and a lot of articles on these topics. Oh, and come to think of it, I have some research experience there too. But one thing I most definitely never will be is a social psychologist. So I will probably never do anything with at least part of my psychology knowledge.
Whoever pays Chuck Klosterman, I guess.
- which animals are and are not rodents.
Most of my other interests fall well short of expertise or at least play into my sideline job as a writer.
I know a thing or two about rifles, though certainly not to the degree that (for example) Martini Enfield does. However, I do know quite a bit about shooting rifles. I’m an NRA-certified rifle instructor, have read just about everything I can on the subject, and have taught lots of people how to shoot. But I have yet to make a penny on it. Quite the contrary, I’ve spent a lot of money on it! 
But I’m not complaining. I enjoy doing it. 
I suspect that that is true of me, as well.
(Although, I suppose I could use my experience here in filling out a resume for being a pre-school recess monitor.)
Trench warfare and PTSD after the First World War.
I know a little about a lot of things, which has never proven all that useful financially until quite recently. Lately, I’ve been winning lots of trivia contests in restaurants and bars, resulting in a lot of almost free meals out. Tonight, for example, I took Mrs. Evil Captor out to dinner and all I paid for was the tip. Nice.
For a moment I thought you were talking about being an expert about knitting in Tudor England.
Cults. I probably know more about Scientology than I do about the Catholic Church (which I grew up in,) and I have an assortment of knowledge about other cults and cult-like groups.
Canadian politics. I think, in eight years of politics and uncountable person-hours I’ve expended over it, I’ve received one (1) small translation contract and one (1) stipended training course. At least the metro got me an internship at one point.
Beyond that, Star Trek and the history of the British monarchy.
Underground/alternative comic books. Stuff you’ve never heard of. Way cool variety of art and storytelling styles.
I seem to have an instinctive knack for childcare. God knows why.
Sunglasses, specifically Oakley sunglasses. I can tell you the available color combinations for every model, including (US) pricing, how much almost any replacement part is, and usually how much it’s worth on Ebay.
Since I don’t work in a sunglass store anymore this is pretty useless knowledge, although I suppose I could become an Ebay dealer if I wanted.
While I confess to having minimal knowledge of Nietzsche, and probably no interest either*, I think reading weird anecdotes about him would be fascinating. Just in case you ever want to start a thread, you know.
- Then again, the way Bricker and others are able to write enthralling reports on subjects I would have sworn I had no interest in, my mind is very likely to change if you give it the opportunity.
Ballroom dance technique (I’m not a particularly good dancer, but I know lots of theory.)
Childbirth and lactation (And here I am stuck in a hospital bed waiting for my C-section - but I know it’s justified!)
The fiction of Anthony Trollope