This service is part of O'Reilly's (a major computer book publisher) Global Network Navigator. With it, one can check a number of exchange rates that are updated weekly. In a neat twist, one can easily calculate one currency relative to any other currency.
Daily exchange rates, for a smaller number of exchange rates and in a slightly less useful format, can be found at the EBB. In addition, the New York Federal Reserve (covered elsewhere in this guide) lists daily rates on a limited number of major currencies.
http://bin.gnn.com/cgi-bin/gnn/currency