I've been working on one to convert US English to UK English. As I've discovered it's actually a lot harder to write something to convert the other way but I hope to get around to providing a reverse conversion one day.
This isn't perfect, but it's not a bad effort (even if I do say so myself). It'll convert most US spellings to UK ones but there are some words where UK English retains the US spelling (e.g. "program" where this refers to computer software). It won't convert words like pants to trousers because my main goal was simply to make the spelling uniform across the whole document.
There are also words such as practice and license where UK English uses either those or practise & licence, depending on whether the word's being used as a verb or a noun. For those two examples the conversion tool will highlight them and an explanatory note pops up on the lower left hand of your screen when you hover your mouse over them. All word patterns which are converted are underlined in red, and the output is shown in a side by side comparison with your original input.
It'll do quite large blocks of text quite quickly, but I prefer to go use it just for a couple of paragraphs at a time - copying them in from a Word doc.
It's still a work in progress so if anyone has any comments or suggestions then I'd appreciate feedback I can use to improve it.
Click where is says "Please try it" near the top of this blog post
http://www.collents.eu/cem/tatep/?x=entry:entry090813-004943