In Photoshop, the Shift, Ctrl, Alt and Tab keys all modify the behaviour of the currently selected tool or action.
Combinations of these keys further modify the behaviour creating a vast array of additional features which you may not know about.
The following list of 53 cool tips demonstrates the above key combinations and some other useful keyboard shortcuts.
http://www.techlicious.tv/2007/10/53-ultimate-tips-for-better.html