Realtime currency webservice

Have you tried with http://openexchangerates.org ?

You can use it's The Forever Free Plan with following features provided.

• Hourly rate updates

• Daily historical data

• 1,000 API requests per month

I would recommend European Central Bank. They provide a daily xml feed, nice and simple.

http://www.ecb.int/stats/eurofxref/eurofxref-daily.xml

You can try Yahoo. It is free and easy to use.

For example, to convert from GBP to EUR: http://download.finance.yahoo.com/d/quotes.csv?s=GBPEUR=X&f=sl1d1t1ba&e=.csv

gives you data in csv format which can easily be parsed.