[lsiebert]

If you are using actual code they write, there are intellectual property concerns to consider.

I think it would be ethical, if you compensated those you didn't hire but who's works you used.

[manishg4]

In one of the companies that I worked, and used to take interviews, the general practice was to ask questions whose answers would benefit us. Also, in certain cases, when we were starting out work on a different domain, we used to call candidates who were experienced in that domain, and get our initial doubts / understanding issues resolved via interviews!