The Currency Data API provides access to rates for every current world currency as well as 4 precious metals: gold, silver, platinum and palladium. You're welcome to get every rate, but few organizations actually need all of them.
NOTE: non-major "exotic" currencies are not traded as much. As such, data is harder to come by, less reliable, and consequently, updated less frequently. Most exotic currencies have a popularity rating of less than 0.2%.