I agree, it's a very emotional topic for people. I don't want people to treat me differently if they knew how much I did or didn't make. Things like anonymous polls and glassdoor give you some idea of the market as a whole, but there is a perception of "bragging" if you make more than the other person.
When my friends and I got jobs after going to college, we disclosed how much we were getting as far as offers go. Of course, we were going across the country, and the rates in Atlanta is not the same as New York or San Fran or Seattle. But we also had the same job during college, and knew how much each other made because of that (fixed pay scale for interns).
As a counter to this, my co-workers and I were buying a retirement gift for another co-worker. We had in mind what we were going to get, but on sudden impulse in the store we decided to upgrade the item. It was mentioned that what we had budgeted wouldn't cover the new item, and a co-worker said "it's OK, the boss [also part of this purchase] can handle it, he drives a Mercedes." It's comments like these that make it awkward to share salary information. Outward displays of wealth have no bearing on income, that's simply what they choose to spend their money (or debt) on.