This plugin is basic but does the job (without all the blotted advertisement like you find in the other currency exchange plugins). Unfortunately, when I first downloaded this plugin back in July 2009 it didn't work. I sent it to my php wizard friend who said: "There were numerous flaws in the PHP script which I fixed, one of them was so unobvious that I bet no one else could have thought of it (well it took me 30 mins too)."
Anyway, my friend fixed the plugin and I contacted the developers via their blog but never heard back from them. If you would like to receive a copy of this fix feel free to contact me at mymarketer.net/contact. Best of luck.