• shift+arrow keys (left, right, up and down) to highlight text
• command+Z = Undo (some programs allow multiple undo)
• command+(X or C or V) = Cut, Copy, or Paste
• command+S = Save a document
• command+P = Print a document
• command+Q = Quit a Program
• command+W = Close a Window
• command+A = Select All in Finder
• fn+delete = Delete one character, pulled in on the right of the cursor
• command+shift+3 = Take a screenshot of the entire screen
• command+shift+4 = Take a screenshot of a specific portion of a screen
• command+F = Find items in a document or open a Find window

Click here for a full list of keyboard shortcuts.