-our

As you know, words like colour, flavour, harbour, honour, humour, labour, neighbour, rumour are spelled differently in the US and the UK.  Oddly, most of these words were spelled -or or -ur until 1066, at which point the French Normans changed the spelling to -our.  Of course, that was 500 years before Columbus re-discovered the Americas, so it is only coincidence that Americans switched back to the "original" spelling, and primarily due to Noah Webster whose 1828 dictionary was less a reflection of current spelling and more a political statement about becoming distinct from the English empire.

Of course, it isn't so simple.  Words like contour, velour, paramour and troubadour retain the u in the US, while words like chancellor, ambassador, emperor, governor, inferior, error, horror, mirror, tenor, terror, and tremor have lost the u in the UK.  America keeps the u in glamour but not in glamorous. Saviour is common in the US. Space Shuttle Endeavour has a u in it as it was named after Captain Cook's ship, HMS Endeavour. 

In the UK, the u is kept in neighbourhood, favourite, and honourable but dropped in honorary, invigorate, and laborious. The u is kept in  humourless but dropped in humorous.  Pearl Harbor is spelled without a u in Britain. "Savory" is an herb similar to sage, and is spelled the same in the US and UK, but the adjective savoury and verb savour have a u in the UK. An arbor (tree) is different than arbour (shelter).

Most interesting of all, though, is that honor is used at the end of the United States Declaration of Independence, even though the US was a colony and long before Webster had his say:

We, therefore, the Representatives of the united States of America, in General Congress, Assembled, appealing to the Supreme Judge of the world for the rectitude of our intentions, do, in the Name, and by Authority of the good People of these Colonies, solemnly publish and declare, That these united Colonies are, and of Right ought to be Free and Independent States; that they are Absolved from all Allegiance to the British Crown, and that all political connection between them and the State of Great Britain, is and ought to be totally dissolved; and that as Free and Independent States, they have full Power to levy War, conclude Peace, contract Alliances, establish Commerce, and to do all other Acts and Things which Independent States may of right do. And for the support of this Declaration, with a firm reliance on the protection of divine Providence, we mutually pledge to each other our Lives, our Fortunes and our sacred Honor.

However, it seems to be a typo--in Jefferson's original draft it is spelled honour!

(Thanks, Wikipedia -- or is it Wikipædia?)