A very useful and quick hint is to use Spotlight on the Mac for currency conversation. Living in the UK and being an Apple geek means that many of the services and applications I use are often priced in US Dollars, so to find a rough estimation of what I'm going to be paying in Pound Sterling I use Spotlight for currency conversation.
Use the keyboard shortcut Command Space (⌘Space), this will display the Spotlight Search Window (img.1).
Then just enter the amount you wish to convert, in this example $25.00 and assuming you're in the UK the conversion to Pound Sterling will automatically display. Other popular currencies will also be shown (img.2).
Quick and easy currency conversation.