CAD tip # 5976:

F1 displays help  
F2 displays text window (Ctrl+F2 since 2014)
F3 switches object snaps  
F4 toggles table mode
F5 toggles isometric planes 
F6 toggles DUCS (switches coordinate display, till A2006)
F7 toggles grid display 
F8 toggles ortho mode
F9 toggles snap
F10 toggles polar tracing
F11 toggles object tracing
F12 toggles dynamic input (2006+)
 
ESC cancel changes, cancel grips
TAB cycles osnaps
SHIFT (on object selection) removes objects
DEL erases objects (hold when pointing)
 
Ctrl+0 toggles clean-screen mode (2004+)  
Ctrl+1 displays Properties window
Ctrl+2 displays DesignCenter window
Ctrl+3 displays Tool palette (2004+)  
Ctrl+4 displays Sheet Set Manager, or Content Manager (2005+, ADT2004)  
Ctrl+5 displays Info Palette, or Project Navigator (2005-2007, ADT2004)  
Ctrl+6 displays dBConnect window
Ctrl+7 displays Markup Manager (2005)  
Ctrl+8 displays Quick Calculator (2006+)  
Ctrl+9 shows/hides Command line (2006+)  
 
Ctrl+A selects all thawed objects (2002)
Ctrl+Shift+A toggles group selection (group/single)
Ctrl+B toggles snap
Ctrl+C copies content to Clipboard
Ctrl+Shift+C copies with reference point
Ctrl+D toggles coordinates display (dynamic UCS, till 2009)
Ctrl+E switches isoplanes
Ctrl+Shift+E pull a region to 3D (PRESSPULL, since 2010)
Ctrl+F toggles osnaps
Ctrl+G toggles grid display 
Ctrl+H toggles Pickstyle (group and hatch selection)
Ctrl+Shift+H toggles Palettes display (2009+) 
Ctrl+I toggles coordinate display (2009+)  
Ctrl+J repeats the last command (Enter)  
Ctrl+K displays the Hyperlink dialog
Ctrl+L toggles Ortho mode
Ctrl+Shift+L adds previously selected objects (2013+)
Ctrl+N starts new drawing
Ctrl+O opens a drawing
Ctrl+P print a drawing
Ctrl+Shift+P toggles Quick Properties display (2009+) 
Ctrl+Q quits AutoCAD (2004+)  
Ctrl+R switches to the next viewport 
Ctrl+S saves drawing
Ctrl+Shift+S saves drawing as (2004+)  
Ctrl+T toggles the Tablet mode
Ctrl+U switches polar tracing 
Ctrl+V pastes the Clipboard contents
Ctrl+Shift+V pastes contents as block 
Ctrl+W toggles object tracing 
Ctrl+X cuts contents to the Clipboard 
Ctrl+Y redoes the undone action
Ctrl+Z undoes the last action 
Ctrl+PgDn switches to the next layout (2004+)  
Ctrl+PgUp switches to the previous layout (2004+)
Ctrl+Home switches to the start tab (2016+)
  
 
Alt+F8 VBA macros
Alt+F11 VBA editor
 
temporary override keys (2006+):
Shift switches ortho mode
Shift+- switches dynamic UCS mode (2007+)  
Shift+) switches object tracing mode (2007+)  
Shift+. switches polar mode  
Shift+A switches osnaps  
Shift+E switches endpoint osnap 
Shift+C switches center osnap
Shift+D disables osnaps and tracing 
Shift+M switches middle osnap
Shift+Q switches object tracing osnap 
Shift+S enables osnap override
Shift+W starts SteeringWheel (A2009+, fixed) 
Shift+X switches polar tracing
Shift+F1 subselection mode (all/off)
Shift+F2 subselection mode (vertices)
Shift+F3 subselection mode (edges)
Shift+F4 subselection mode (faces)
Shift+F5 subselection mode (history in compound objects)
 
CTRL+mouse cycles selection of overlapping objects (till 2006)  
Shift+space cycles selection of overlapping objects in 2D (2007+)  
CTRL+space cycles selection of overlapping sub-objects in 3D (2007+)
CTRL+ALT extrudes the selected region into 3D - PRESSPULL (2007-2009)  
CTRL+arrow moves cursor
Arrow up/down - command history
 
CTRL+SHIFT+letter goes to the property in the Properties window  
ALT+down arrow opens a list in the Properties window
ALT+up arrow closes a list in the Properties window