Organisation is used in Britain and other countries that were controlled by the United Kingdom, so basically most English-speaking countries use this spelling. Organization is used only in the US (maybe Canada but I'm not sure).
Also see this in wikipedia:
"American spelling accepts only -ize endings in most cases, such as organize, recognize, and realize. British usage accepts both -ize and the more French-looking -ise (organise, recognise, realise).The -ize spelling is preferred by some authoritative British sources including the Oxford English Dictionary — which, until recently, did not list the -ise form of many words, even as an alternative — and Fowler's Modern English Usage. The OED firmly deprecates usage of "-ise", stating, "[T]he suffix…, whatever the element to which it is added, is in its origin the Gr[eek] -ιζειν, L[atin] -izāre; and, as the pronunciation is also with z, there is no reason why in English the special French spelling in -iser should be followed, in opposition to that which is at once etymological and phonetic." Noah Webster rejected -ise for the same reasons..."