There are so many - I do actually have a file on my computer called 'English to American Conversion' and I still keep finding new differences to add to it, so much so that I swear I'm writing in a foreign language when I write Lois & Clark fic. Anyway, looking through the file, some of the more common ones and ones I know I've had to change are (English = American):
Autumn = Fall
Biscuit = Cookie
(Car) Boot = Trunk
Chips = Fries
Cinema = Movie Theatre (unfortunately, while I'm happy to change the word, not so the spelling! wink )
Cot = Crib
Crisps = Chips
CV = Resume
Flat = Apartment
Holiday = Vacation
Jam = Jelly
Jelly = Jello
Knickers = Panties (although I have a note on this one that it's my assumption as I'd never heard of 'panties' before I came across nfic!)
Lift = Elevator
Mobile = Cell Phone
Nappy = Diaper
Pavement = Sidewalk
Petrol = Gas
Porridge = Oatmeal
Rubber = Eraser
Rubbish = Garbage or Trash
Shop = Store
Sweets = Candy
Torch = Flashlight (still confuses me - why would you use a flashing light to see in the dark?)
Trainer = Sneaker
Trousers = Pants
Althought, I've just noticed that I've missed out the English meaning of 'pants', so I'll add that in.
Pants = Underwear.
And, believe me, that's just a snapshot of the table. It goes onto page 3 at the moment, and is ever-growing. Please let me know if I've got any wrong, it'll make my beta reader's job easier.