The OANDA Currency Converter is one such easy-to-use site. Not only can see the current currency exchange rates, but you can also view historical information. The site allows you to access currency exchange rates as far back as January 1990.
Fun fact: Today, one US dollar is equal to:
£0.64533 (British Pound, GBP)
¥76.42 (Japanese Yen, JPY)
₩1,179.80 (South Korean Won, KRW)