Getting cross-browser AJAX request working for our currency convertion service was the biggest challenge I had. After Googling everything under the sun, I ended up adopting the Yahoo webservices way of doing things - use JSON.

Have a look and let me know what you think (Convert, buy, transfer and compare currency (money) exchange rates.). We wanted to create a simple converter for webmasters to use on their website, and I think we might have cracked it.