What liscense plates have you seen? What are your favorites? Least favorites? Me, I have an anecdote about this. I see Canadian plates a lot, most all from Ontario or Quebec. I had never seen a Canadian, non-Ontario or Quebec liscence plate when this story happened. That day, I told my cousin that by year’s end, I would see a Canadian liscence plate not from Ontario or Quebec. Mere moments after I said that, I saw a white pickup truck… bearing Alberta plates.
I have seen Mexican plates exactly twice in Toronto.
I’ve been to dozens of countries and seen license plates of course. Perhaps the oddest… an Oregon, USA plate in Prague, Czech Republic. Don’t think they drove there. 
Slightly off topic, but North Carolina is changing its licence plates, from attractive blue letters to ugly red ones, and I got my new plate today. They are also issuing new numbers, and mine now starts with “XVD”, which looks like I’m proclaiming to the world that Hey! I’m now STD-free!, thank you very much DMV…
Someone at my former workplace had a personalized plate that read “28 IF.”
I thought that was pretty cool.
Then, of course, there’s the legendary “3M TA3”.
On a spring break trip to Florida (from New York,) we saw a plethora of Ontario plates. Far more than should be normal, especially since so many were in the southern US. Our theory was that Ontario was going to invade America, but had the map upside-down, and thought that Florida was on the “top” and bordered Ontario. 
You mean you’re not? Just kidding - fellow Tar Heel. BTW tag office gets a box of consecutive numbered plates from the state prison (they make them with conviction) with the same alpha prefix. If you research this by seeing what they are giving out at that time at “your” distro center and don’t like it, you can either wait until a new series arrives or go to a different tag issuing office, probably in another county, and get a different prefix. They also apparently reserve the OBX prefix for distribution on the Outer Banks for which that is a mini acronym.
In my former home seaside tourist town, a little old lady would sit in a chair on the roadside every day during the summer tourist season and write down the different license plates she saw. I don’t know how finely she filtered them, probably just by state.
Since Purdue attracts students and faculty from all over the world, I see a lot of fairly exotic license plates. In the past year, I’ve spotted a Puerto Rico tag, a Hawaii plate, and a car registered in the Mexican state of Jalisco. Such states as New Mexico, Arkansas, and Alaska are better-represented around here than you’d probably imagine.
I’ve been to Saskatchewan, Manitoba, Ontario, and Quebec, so I’ve obviously seen plenty of plates from those provinces, as well as several from British Columbia, Alberta, New Brunswick, and Nova Scotia. I can recall seeing exactly one “Newfoundland and Labrador” tag (on a family vacation when I was a kid), but I’ve so far been shut out by Prince Edward Island and the Canadian territories. Too bad – ever since I first saw a picture of one, my favorite plate design has been the bear of the Northwest Territories.
In this case you have no choice. This year, when you mail in your annual licence renewal check, instead of just sending a new year sticker to affix to your current plate, they mail you a completely new plate.
I’ve seen a Guam plate while driving through Jacksonville, Florida. I’m sure it had to be the result of a military move. In Dallas, there are also a couple of cars, parked near Union Station, with tags which appear to be from an Asian country. I’ve tried to Google to see what country might have a consulate near there, but haven’t had any luck.
The absolute strangest license plate sighting was this summer on Visingsö, a small island in the middle of a lake in Sweden. It has only a handful of streets (I don’t know if there are even any street names) and about 800 residents. We went there with some friends who live in Jönköping, and we were speaking only Swedish and in a very Swedish mood.
Going down a street in a horse carriage, what do we see but a car with Maine (US) plates. We currently live in New Hampshire for 11 months of the year, so it was very weird to see a “neighbor” there.