another very great free and opensource link is this:
https://raw.github.com/currencybot/open-exchange-rates/master/latest.json
(I found about it here: http://josscrowcroft.github.com/open-exchange-rates/)
[Update]:
Open Exchange Rates project data has been moved away from GitHub.
It is available now at: http://openexchangerates.org/
Data in JSON format is available at: http://openexchangerates.org/latest.json
No access fees, no rate limits, No ugly XML - just free, hourly updated exchange rates in JSON format.
This is not "entirely" free now. The new licensing states that up to 1000 hits per month is allowed, and then you need to pay. You also need to pay if you want to use the single currency converter (basic functionality).
[ Note: You may want to look at this answer as well. ]
