There are a few currency converters out there but my favourite is XE.com - Universal Currency Converter. It is said to be the World's most popular currency tool.
According to the site:
"You can perform interactive foreign exchange (FX, or forex) rate calculations, using live, up-to-the-minute currency rates"
Of course, you can also use the Google search engine. For example you want to find out how much US$15 is in pounds. Just type 15 USD in GBP and you get the answer - it is 7.49437922 British pounds.