You didn't specify whether you wanted raw data, or just a website to look it up on. I use XE's currency charts to find what I need; it provides exchange rates to the hour, back for at least a few weeks, and to the quarter-hour in the past few days, for free.
I do wish I had a source of raw data, so I could write a script and just type the date and time I need. I had a quick look at the HTTP responses that page uses and didn't find anything easy, only a long string of alphanumeric characters. I assume the code that interprets it is too obfuscated for it to be worth my time deciphering it, and that doing so would violate their terms of service. So I just mutter as I zoom and scroll around the graph, hover the mouse pointer, and memorise the numerals.