Most people know that Command-S ( Ctrl-S for Windows) saves a document and is much faster then taking your hands off the keyboard to reach for the mouse/trackpad.
Here are a few common ones I use when editing documents in Photoshop.
New…Command-N (for Command use Ctrl for Windows users)
Layer via Copy…Command-J (feels like my most often used command)
Merge Visible to new Layer…Shift-Option-Commamd-E (Option is Alt in Windows)
Spot Healing Brush…J (Toggle to different heal brushes Shift-J)
Some of these options are also programmed into my Wacom Tablet and Pen to speed up my work flow. The key is to just find a few things you do a lot and then learn the shortcut for them, its amazing how much time you can fine yourself saving.