Now this can be a condiment or an actual topping, so lets hear em and let us know if it was gross or just sounded gross. :P
 
For me, probably would be Pastrami. Not sure why it was there, but damn it was tasty. I've seen a burger with peanut butter on it, but I wasn't brave enough to try it.

Only fries. I don't experiment with burgers much. Wouldn't want to ruin a good thing.

" Only fries. I don't experiment with burgers much. Wouldn't want to ruin a good thing. "

Oh boy, I had a burger in Vegas that had STEAK on it. Yes, its true, my mind imploded at first as well.

Hummus & maybe jizz

@TheSilentTruth said:

" Only fries. I don't experiment with burgers much. Wouldn't want to ruin a good thing. "

Your avy = Nyuu/Lucy right? "

Cherry Cricket here in Denver is famous for cream cheese and freshly diced jalapenos. Best fucking burger I've ever had!

Probably pineapple. Believe I had mushrooms once as well.

in new zealand i had a burger from fergburger called the bun laden, that had falafel patties, dressed with a lemon yoghurt and a chipotle chili sauce, avocado, aioli and all the trimmings in it. 
 
it wasn't very nice, but i tried some others and fergburgers are the best burger chain in the entire world.

a half pound of fried cheeze

I know TGIF used to have the cheesy bacon cheeseburger which has mozzarella sticks on it.  Not really strange, but it was delicious.

Beetroot, I guess? My Dad actually likes it on his as do a few people down here but I never really understood why.
I've had pineapple in a burger before which was strange too. A fried egg might be considered strange as well but I actually like that one from time to time.

" Cherry Cricket here in Denver is famous for cream cheese and freshly diced jalapenos. Best fucking burger I've ever had! "

The most epic hamburger topping i've ever had would have to be a chicken burger  in the form of the McGangBang

A Pineapple. It doesn't work IMO. Especially if its cold. Its a sharp contrast to the warm/hot burger. Pineapple is passable on a pizza but not in a burger.

peanut butter

Here's a surprise probably, I've never had a hamburger or a cheeseburger or any variation.

Pineapple, peanut butter, and teriyaki sauce. It was this small diner in Menomonie, WI and on the menu was the "Yuck Burger" It was really damn good, the pineapple was grilled, and it was a peanut butter/sauce blend.

Sweet n' Sour sauce.  A surprisingly good combination.

Foie gras. Delicious.

Pineapple, I guess, but it was tasty. In my mind, the strangest burger topping is mayonnaise, because fuck mayonnaise.

I like pineapple on my hamburgers.

Eggs

Hmm, nothing too exotic but a fried egg is always a good choice but I guess it's getting more commonplace now. 

Avacado

I have never eaten a burger with anything more than just ketchup on it (I've bitten into such burgers, but never proceeded to swallow any of it).  So ketchup kind of is by default.

A fried egg and it was delicious.

Never had anything that strange but the most adventurous was probably an avocado. 

Burgermeister near where I live has a Tropical Burger on which they put pineapple and mango salsa. It's actually a little underwhelming, sadly. They also have a teriyaki burger that's much better, but I'm definitely gonna have to try that cream cheese/jalepeno pepper one. That sounds incredible.

The weirdest topping I have ever had on a burger was an egg, and It was quite delicious.