Are you familiar with the Millionaires' problem? [1]

There's an easy solution when you're looking for an average salary (and everyone doesn't mind doing a little bit of calculation). The first person thinks of a random number, adds his salary to it and tells it to the person on his/her right (so noone else hears!). The next one adds his salary and tells it to the next person and so on. When it re-reaches the first person, he then subtracts the original random number. The result is the sum of their salaries, which can then be divided to yield an average. All done in a way that no one person learns what the salary of any other person was.

[1]: http://en.wikipedia.org/wiki/Yaos_Millionaires_Problem (link sadly broken by HN url parsing?)