Currency Monitor is a free gadget that will let you compare a pair of currencies, even convert any amount.
Please note: the gadget appears broken! You can try using instead the Currency Gadget.
The gadget will let you enlarge it, in case the usual size is not convenient enough for you. It will also let you define alerts following a currency’s growth.
To change currencies, just click on the default selection (Euro / USD) and choose a different pair.