(answered by: alexcwlin; edited by: Adam Lam)
This is the most common English versus British spellings (exceptions not included).
1. American “-or” versus British “-our” spellings:
American: color, flavor, behavior, harbor, honor, humor, labor, neighbor, rumor, splendor;
British: colour, flavour, behaviour, harbour, honour, humour, labour, neighbour, rumour, splendour.
2. American “-er” versus British “-re” spellings:
American: caliber, center, fiber, liter, luster, manoeuver, meager, meter, saber, somber, specter,
British: calibre, centre, fibre, litre, lustre, manoeuvre, meagre, metre, sabre, sombre, spectre, theatre.
3. American “-se” versus British “-ce”:
American: license, practise, defense, offense,
British: licence, practice, defence, offence.
4. American “-ize” versus British “-ise”:
American: realize, recognize, organize;
British: realise, recognise, organise.
5. American “-yze” versus British “-yse”:
American: analyze, paralyze, catalyze;
British: analyse, paralyse, catalyse.
6. American “-ll” versus British “-l":
American: distill, instill, enthrall, enroll.
British: distil, instil, enthral, enrol.
(“fulfill” is double “l” in both American and British English).
There are plenty more and these are only the more common ones.