One of the biggest problem is the difference in the shift key sequence:
PC: Ctrl, Win, Alt.
Mac: Ctrl, Alt/Option, Command
By default, "Command" maps to "Win", so Alt and Command are swapped.
I don't care about key labels, in fact, I am using a Das Keyboard Ultimate, that goes without labels.
I want some simple/easy-to-configure app that will do remapping when I attach an external keyboard like this, but will not change Mac keyboard (or make it really easy to switch).
Upd.: solved with KeyRemap4MacBook.