XE.com offers many online foreign exchange tools, including a convenient table of cross rates, e.g., the ratios of one currency to another, for frequently consulted currencies. Live mid-market rates are refreshed once a minute. You can also create cross rate tables based in currencies of your choice (even obsolete currencies) using current or historical rates.

The site has a quick currency converter, but you can access a "full" version of the Universal Currency Converter, which contains every known world currency: over 180 currencies in over 250 geographical locations.