NB. Some of these seem like little differences, but you'd be surprised how much confusion they can cause!
Purple=English; Blue=American; Orange=Australian
Duvet-Comfortor-Dooner (possibly just New Zealand)
Chips-Fries-Chips/Fries (fries in fast food stores, but they sell oven chips)
Crisps-Chips-Crisps (sometimes chips, I think)
Tea (like, regular tea)-?-English Breakfast tea
Flip-Flops-Flip-Flops (though a couple of generations older use the Australian word)-Thongs
Sweets-Candy (EVERYTHING is candy - chocolate, sweets, everything!)-Lollies
Lollipop Stick (from an ice cream, like a Magnum)-Popsicle-Paddle Pop Stick
(Ice) Lolly-Ice cream bar-Ice block (?)
Surname-Last Name-Surname/Family Name
Felt Tip Pen-Felt Tip Pen-Texta
Kitchen Roll-Paper Towels-Kitchen Roll
Hot dog-Hot dog-Sausage
Pavement (as in by the side of the road)-Sidewalk-Path? (thanks to Ben Hession)
Car Park-Parking garage (also pronounced "gar-AHge")-Car Park
Coriander-Cilantro-Coriander (thanks Roxanne Martelli!)
Rubbish-Garbage-Trash (this one has gotten into my head... thanks for the reminder Ben Hession!)
Trousers-Pants-Trousers? (again, thanks to Ben Hession for the reminder! My flatmate had no idea what 'trousers' were!)
*Rag is a general term used for all kinds of cloths found in the kitchen.
(Thanks to Mallory Miller, of Washington State, for reminding me of many of these!)
A few things the English say that most Americans at least seem to find hilarious:The loo (the bathroom or toilet)
Cheers/Ta (thank you - not just when clinking champagne!)
Some intriguing Australian phrases:Cool as!
Super [insert any noun here]. Often 'super cool' or 'super good'.
So good! (possibly American in origin)
Tagging the end of any sentence with 'hey?' to make it a question. Everyone picks this up sooner or later.
Heaps (heaps cool, thanks heaps - just means 'loads'!)
(Possibly American in origin but both of these have done the rounds of College): STOKED! (all rights reserved for Roxanne Martelli) (means ridiculously excited for something coming up).
All you College/Church people/Australians in general out there: get posting with additions to the list!