BTConvert is a portable, open source, currency converter that automatically refreshes currency rates every 30 minutes. If supports conversions between Bitcoin (BTC), USD, Euros, GBP and AUD. I wrote it in AutoIt and it uses Google and Mt. Gox to get the currency rates. Its usage is fairly simple, just edit a field and press enter to convert it to all other currencies. There is also a thread in the Bitcoin forum about it.
Download BTConvert (v3) from here.