flavour vs flavor
behaviour vs behavior
rumour vs rumor
honour vs honor
armour vs armor
neighbour vs neighbor
splendour vs splendor
Samuel Johnson’s famous dictionary (1755) went a long way toward standardising British spelling. Before that, it was pretty inconsistent, often showing more than one rendition of the same word within a single passage. The -our endings show the effect of the Norman conquest, which introduced French words and spellings to English terms.
Noah Webster’s English dictionary (1828) likewise brought standardization to American English, and there were some politics involved too, as he sought to simplify some words and also to deliberately promote variance from customary British forms.