Open Exchange Rates is a free, real-time JSON API that provides exchange rates (forex/fx rates) for 150+ currencies, relative to USD.
Rates are collected from several reliable sources and blending together algorithmically, to acheive a consistent and unbiased level of cross-market accuracy.
You can use these data anywhere and in any language - whether performing JavaScript currency conversion on the client-side (eg. for a web app, smartphone app or online shopping cart - try money.js) or for back-end processing (eg. databases, dashboards, analytics, accounting, tickers, desktop software, whatever).
The latest exchange rates are available on openexchangerates.org, along with historical EOD (end of day) rates going back to January 1999, with HTTPS support, optional JSONP callback and friendly Access-Control HTTP headers - so you can load it in via AJAX with a cross-domain request without worrying about browser security restrictions.
To get the data, you just need to access latest.json via AJAX, CURL, Scrapy, node-httpagent, or your method of choice. There's a bunch of documentation and examples/sample projects are coming to this repo very soon.
You can sign up easily at the signup page - any questions, send Joss an email!
As with all exchange rate data, accuracy can never be guaranteed when you're not paying through the teeth for the service - and when money changes currencies, everyone takes a cut - so it's a good idea to inform people that these are for informational purposes only, something like "Converted prices/exchange rates are for informational purposes only." Feel free to say that rates come from the Open Exchange Rates service.
Check out openexchangerates.org for all the info.
There are guides for most languages, with more on the way. If you're working in client-side or server-side Javascript, check out money.js, a tiny (1kb) JavaScript currency conversion library.
There's a License, Terms and Conditions and Privacy Policy available on openexchangerates.org, which you are required to read and understand before querying the data.
This Git repository will soon be updated with working code samples and example projects for using the API and data.