Music, post-1930 or so. Especially from the 60s to now. It’s my life. I could set the record for continuous waffling on the subject.
Another one is my own music, which is especially boring for whoever I’m talking to because not many people have heard what I’m working on (yet) so often they don’t know what the music I’m waffling on about even sounds like.
Please note my user name
Vampires. I could go on for hours about various vampire myths. At one time I wanted to put together an encyclopedia centered around vampire myths.
Cartoons. I never lost the love of cartoons I developed as a kid, and even today it’s one of my favorite subjects.
Bad business decisions past bosses have made, and how they never listen to their employees about how to change things for the better.
The uselessness of belief. I’ve stopped myself mid sentence before.
How much I hate the penny. Don’t get me started.
WoW.
Cats.
Climatology, and how capitalism has ruined everything. 
The tsunami.
Hong Kong.
China.
ETA: east Asia in general.
Agriculture/gardening/food production. The only person who wants to hear it these days is my 95 yr old grandfather. Everybody else just nods politely. 
Brazil, the Portuguese language, Brazilian music
Gadgets
Home improvement (Let me show my latest project!)
Politics. For a certain amount of time after any long trip, the trip.
Hockey (surprise!)
Cooking/Eating
Makeup
My dogs
Las Vegas
I don’t really talk much though. One on one, you are likely to find me boring because I don’t find the need to speak all that often. I’d rather be part of a group of 3 or 4 people and listen. Just like here. I read the board (and a couple others) every day, but only occasionally comment or start a thread.
Sailing … arg, when I was in the business I could not stand people talking about it endlessly. Now I very rarely sail but have friends who are new enthusiasts, and they **want **to hear salty tales - stop egging me on friends. My eyes are glazing over and yet … I’m the one talking.
U.S. immigration issues.
Not me, but a female friend. Whenever I meet her for lunch or whatever, I know I am going to hear the following two sentences come out of her mouth, which MUST be answered before we can move on:
Better seat us on either end of the table, because except for the bills and collections part, yeah, me too.
Also, brain injury, disability rights and assistive technology. I’m a regular cheery little ray of sunshine.
Apes. This usually comes up when someone asks me what I want to do with my life (work with apes) and they start asking questions. I try to keep it short, but then someone asks a question like, “what kinds of things are threatening them?” or “did you hear about the chimpanzee that ripped that lady’s face off?” and suddenly it’s ape hour.
Motorcycles.
Auto racing (mainly F1 and sports cars, but any form to some extent), especially history and technology.
Horses. From the utter joy of watching a newborn baby get to his feet, to the ethics of breeding horses with obvious genetic disorders, to just about anything.