I recently saw a version of the Euthyphro dilemma that to me resonates better than the usual presentation. It's the same argument, but it fits better as a reply when theists do the old "can't be moral without God" dance.
In a comment by Ian Spedding at John Wilkins blog, he said:
"Ask them if they believe their chosen deity is a capricious being or one of reason and order. If the former, why follow a moral code that was thought up on a whim, if the latter, what is to prevent us from reasoning to the same conclusions all by ourselves."
I have a feeling it may be further teased along, but it's a good one as is.
(Some of the other discussion in that comment thread is also interesting.)
[edit: b0rken link fixx0red]