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Regional abbreviations:
US - United States (of America)
CA - Canada
UK - United Kingdom (Britain, England)
AUS - Australia (Oceania included)
NZ - New Zealand
Suggest better codes if you know them, and if anyone knows any other regions that speak english, feel free to add them, as well.

Clothing/Cloth

Washcloth (US), Flannel (UK)
n. - a small swatch of cloth used for washing, especially washing the face. Flannel in US reffers to a specific type of cloth or pattern.

Pants (US, CA, AUS), Trousers (UK)
n. - an item of clothing that covers the part of the body from the waist to the ankles or, sometimes, the knees, each leg having a separate tubular piece.

Cars and Automobiles

Hood (US), Bonnet (UK)
n. - (automotive) engine cover: the hinged cover over the engine of a car or other vehicle

Trunk (US), Boot (UK)
n. - (automotive) storage compartment: an enclosed storage compartment in an automobile, usually at the rear.

Gas (US), Petrol (UK)
n. - A thick, viscous fluid used in automobiles to drive the pistons. "Gas" is short for "gasoline," and "Petrol" is short for "petroleum."

Buildings and Locations

Elevator (US), Lift (UK)
n. - A small "room" operated by pulleys that moves up and down to transport passengers.

Apartment (US), Flat (UK)
n. - home in larger building: a self-contained residence, situated with other similar units in a larger building.

Food and Drink

Cola, Soda, or Pop (Unknown at present)
n. - soft drink: a flavored and carbonated drink, served cold
(extremely variant; can change depending on speaker more than region)

Ketchup (US), Tomato Sauce (AUS)
n. - A tomato product similar to paste (slightly more liquid, however) that is often used for flavoring or garnish. Also "Catsup," but that might be a different product altogether. Need assistance.

Sub/Hoagie/Grinder (???)
n. - a long sandwich, made in a deli with deli meat and often cheese. "Hoagie" is used primarily in Philadelphia and New Jersey, and "Sub" is used in the rest of the country. Grinder for New England. Assistance, please. Need UK, AUS imput!

[French] Fries (US), Chips (UK)
n. - deep-fried potato strips: thin strips of potato deep-fried in fat. In the US, "chips" refers to flat chips rather than strips.
Interesting trivia: "French" fries were invented in Belgium.

Other/Random

Invigilator (UK), Proctor (US)
n. - One who watches over an exam.

Knackered (UK), Tired (US)
adj. - Exhausted or needing sleep

Kip (UK), Sleep (US)
v. - to not be awake; to be partially or fully unconcious.

Cigarette (US), f** (UK)
n. - tobacco within a rolling paper

Bugger (UK), Jerk (US)
n. - a slang word with many translations, this mostly means someone annoying, rude, or otherwise unsavory.

Let's get some more Canadians and Austrailians to contribute! If you happen to live somewhere that speaks English, you probably know something about something.

The MoUsY spell-checker suggested an excellent blog with this subject - I'll be using that to help, of course. The link is here, I think.

This would probably fall more under colloquial slang, but...

Cola, Soda, or Pop: All three refer to carbonated drinks, depending on where in the US you are. I've actually been in towns where all carbonated drinks are universally known as "Co-cola"s.

This is only a US regional difference, really, but a long sandwich with deli meat and cheese varies in name depending on where you are. It's a hoagie around Philly and NJ, a sub elsewhere, and a grinder in New England. (It's random, but there you go.)
Also along the food route, don't forget the fries vs. chips. What the US thinks of as french fries, the British call chips! =)

How about "ketchup" (US) vs. "tomato sauce" (AU)? Not sure about UK though.

In Hong Kong, most people are taught BrE, but "ketchup" is more common than "tomato sauce". (Probably because it sounds similar to the Cantonese word, but that's getting off-topic.)

In Australia (note the spelling), I'm pretty sure that "pants" and "trousers" are used interchangeably.

There's "invigilator" (UK) vs. "proctor" (US). That is, a person who watches over an exam. In Australia, I've heard someone say "invigilator", but usually it's "supervisor".

If you're interested in regional differences, I recommend this blog:Separated by a Common Language.

Wikipedia also has a list of AmE and BrE differences.

On this topic, I'm doing a survey about what you call this drink. I call it an "iced chocolate", but my friends in the US had no idea what I was talking about when I said that. What do you guys call it? (If you answer, please tell me which country you're from too.)

The drink in the picture is made of milk, chocolate syrup, vanilla ice-cream, and cream.

Some of my friends from the US called it a "chocolate milkshake", but at least over here in Sydney, "milkshakes" are blended.

I'm going to have a chocolate milkshake later, and I'll post a picture of that too.

By the way, there's only one i in "Australia".

Yeah, that's what I'd call it...

A lot of it might include how you were raised, I think.

For example, I was born in America and raised very close to Canada. I maintain an accent that hints Canadian, but that also hints Midwest. Most can get around it.

It's my vocabulary and spelling that can confuse people.

My best friend had to point out to me that people didn't understand what I meant when I said "I'm knackered" or "bugger". Oddly, I've never been to England. Why then, she wondered, did I use the words?

My next-door neighbor - practically family to me - apparently lived in England for the first thirty years of her life. Because I spent so much time with her, I picked up on her vocabulary; additionally, I believe my spelling came from reading too much at her house.

So I've grown up using terms no one can understand and spelling things in ways that confuse my friends. I kip, complain about people smoking fags, and my favourite colour is purple.

I'm a fine example.