For those who travel out of the country, this extension is a great assistant with a Portable Firefox on a thumbdrive or laptop.
The Exch extension uses Yahoo! Finance (http://finance.yahoo.com/currency) for its data and the extension is always up-to-date with currency exchanges. The extension (as does Yahoo!) supports over 150 countries for currency conversions.
Exch uses the sidebar to compute its values and gives and immediate response back, so it is quick.
The extension is very handy when you want to do a conversion, but again, if you don't travel, this extension may not be beckoning you to install it.