I recently asked the question "what are best questions to ask references of people you are interviewing?" over Facebook, LinkedIn, and Twitter and here are some of the responses I got:

AS:
"If you were in my shoes, would you hire this person?"

RF:
What advice would you give me as a future manager this person?

ET:
What are the major accomplishments of the candidate? Are there any areas that are development opportunities for the candidate?

SV:
Based on what you know of this person's desires and capabilities, what do you see him/her doing in 3-5 years?

AC:
(other than making sure to contact references other than the ones they gave you) – how about "do you trust this person's integrity, capacity, and intellectual curiosity?"

MC:
does he know if he has 5 pairs of white socks and 5 pairs of black socks, and he HAS to get dressed in the dark… how many socks does he have to pick to actually find a pair?

or just, would you work with this person again???

SM:
Are you friends outside of the work?

LK:
Why do you no longer work together? And how come you haven't hired him/her already.

MY:
is there anything else that I should know about so and so.. last question.. the truth comes out usually if they have been lying..

SP:
I'll pay you $500 if you save me from not hiring this person. A great person will have sufficiently impressed that reference so that they would reject this.

AM:
What kind of a drunk is he/she?

JT:
i always ask if the reference would hire that person again to work directly for them. usually gets a good response.