I found a neat site that gives the meanings of each individual card in each position of a standard Celtic Cross spread - when I looked back at old readings I had done where the outcome is known, I found them very accurate:
Position of cards in Celtic Cross spread (and a few suggestions for other spreads as well).
Update 2019: the original site is gone but it was archived by archive.org, so I have linked to the archived pages, because this was such a good resource. I believe if webmasters want they can have archived copies deleted, and it is a shame for this to be lost.