I'm using Yahoo Finance API like this:
http://download.finance.yahoo.com/d/quotes.csv?s=GBPEUR=X&f=l1
That gives me the current rates for GBP -> Euro.
But how would I get it to list all available currencies?
http://download.finance.yahoo.com/d/quotes.csv?s=X&f=n0 and http://download.finance.yahoo.com/d/quotes.csv?s=X&f=c4` is a start, but I can't get further than that.
Documentation: http://code.google.com/p/yahoo-finance-managed/wiki/csvQuotesDownload
There is no official list of currencies/stocks supported by Yahoo Finance API.
You can either go to the currency page, copy paste all the trading pairs and use them directly or use this xml, parse it and retrieve both names and prices.