Exchange Rates is a free desktop gadget that will let you convert several currencies or monitor constantly a pair of currencies, being also connected with the On Line Currency Converter for more currencies.
Note that the gadget appears to be broken, please get instead the free Currency Gadget.
Digits are somehow small, but a new version provides larger fonts, along with support for Thai Baht. If you need these changes you are welcome to download the New Exchange Rates gadget.