atlas marker used to have alt+shift+m, but it was removed. You can add it again, if you desire it (I tend to).
See the NGUIMenu.cs file where it says
[MenuItem("NGUI/Open/Atlas Maker", false, 9)]
[MenuItem("Assets/NGUI/Open Atlas Maker", false, 0)]
and add "&#m" after it says "Maker.
[MenuItem("NGUI/Open/Atlas Maker &#m", false, 9)]
The problem is that these shortcuts are global, so when other plugins or unity itself wants to use a certain shortcut, you might get confused as to which function, if not all of them, get called.