I found many, but none of them were too smart. The main problem was that they required me to leave the page I was on, or open a new window. And then, of course, I had to type in the value myself, and the next time I used it, I had to select my target currency, even though I, as an American, usually wanted to convert currencies into dollar units.
So here it is: Joel's currency conversion browser button. Features include:
This applet uses perl to get the latest currency tables, mysql to store them, and javascript for the calculations, just in case you're interested in that sort of thing ;) It is not affiliated with the Bank of Canada in any way, except that they were kind enough to let me use their data. No warranty is stated nor implied as to the fitness of this application ... blah ... blah ... blah ... nor shall I be liable ... blah ... blah ... contents may settle during shipping ... blah.