I am new to Mac OS X. Is there any good desktop app or dashboard widget which displays latest exchange rates like Windows 7 gadgets on right side.
Calculator.app doesn't display the exchange rates, but it does allow you to convert between them. You can do this by entering a number in the calculator, and then going to the Convert > Currency... menu and choosing the appropriate currencies.
If you're looking for a Dashboard widget to show exchange rates, I'd suggest this one: http://www.apple.com/downloads/dashboard/calculate_convert/currencyconverter_palplesoftware.html