You can use Currency Watch, a free and safe desktop gadget, to monitor the exchange rates of several pairs of currencies simultaneously.
This gadget provides a link to an on-line converter, just in case you need also to make conversions of custom amounts. Currency Watch is a form of currency meter that takes advantage of this On-Line Currency Converter.
Please note that if you downloaded the gadget before May 5th, 2015, it may not work now. A new version (2.5) is available that fixes the problem.
If your conversion needs are demanding, you can get also a complementary gadget, the New Currency Converter, a free, safe and working desktop currency gadget dedicated to conversions, or a similar pretty nice and free gadget called Exchange Rates.