Difference in Salary amongst colleagues.

nyermen

As others, same observations, but I would add, I've noticed over the years that many companies think they can get away with it (and the small ones then squeal when you quit, the big ones factor in that all the long term staff savings outweigh the loss of experience from those who quit).  
Then if you do resign, suddenly its all "oh no don't go, we can offer you 15k more".  If I was in that position, my view would be "too late!".