This is a simple currency converter based on rates from the European Central Bank - more about this on my blog.
What I really wanted was a minimally simple API somewhere for updating an application's exchange rates. If you try out a currency conversion on this page, the result will include a link to a plain text response from spider.my which contains nothing but the rate used in the conversion.
Updated 22nd Feb 2010 to use javascript to show the exchange rate (for the selected currencies) beside the Convert button. Also added link to XML response for javascript.