< = Previous Frame
> = Next Frame
Enter = Play
Ctrl + Enter = Play whole movie
Ctrl + Alt + Enter = Play Scene
F5 = Insert frame: double the frame you are currently selecting
F6 = Insert keyframe: goes to the next frame while keeping the same thing you currently have on the layer you are on.
F7 = Insert blank keyframe: same as above, but empties the next frame for you (only on the layer you are currently selecting)
F8 = New Symbol
F9 = action panel (for coding)

MORE STUFF

Selection (V): Selects objects in your scene
Sub-selection (A): Selects the perimeter of your object. It's basically a really in-depth free transform tool
Free Transform (Q): Transforms your object either by distortion, skew, scale, and actual free transformation.
Gradient Transformation (F): Selects and directs which way or what position the gradient of which your object is embedded with.
Line (N): Creates a line that is 1 dimensioned in properties. <-- most useful shortcut if you're a line animator
Lasso (L): Selects specific sections of your object
Pen (P): Ridiculous method of creating lines through forms of math. Don't use it. You'll only get lost.
Text (T): Creates a text with modifiable properties
Ova (O): Creates an oval using line as borderlines and brush as the filler.
Rectangle/PolyStar (R): Creates a rectangle/polystar using line as borderlines and brush as the filler.
Pencil (Y): Free hand way of drawing a line
Brush (B): Free hand way of drawing a 2 dimensional shape. <-- most useful shortcut, if you're a brush animator
Ink Bottle (S): Makes one line a different color
Paint Bucket (K): Makes one shape a different color
Eyedropper (I): Takes colors of a shape or line and making them your color swatch of choice.
Eraser (E): It does its job.

EVEN MORE STUFF (RawGreen did all of this)

Shift + F5
Remove row of frames
Remove selected frames

Shift + F6 + Selected Frame(s)
Clear Keyframe / Clear Frame

Alt + M + T + T (or use; Ctrl + Shift + Z)
Remove Transformation

Alt + M + T + H
Transform Horizontally

Alt + M + T + V
Transform Vertically

Ctrl + Alt + M
Mute all sounds from playing in timeline
(handy if you want to listen to other music while scrubbing in the timeline)


SIMPLE ONES YOU MAY NOT KNOW (Again, thanks RawGreen!)

Ctrl + X = Cut (removes the selected to your pasteboard)
Ctrl + C = Copy (copies the selected to your pasteboard)
Ctrl + V = Paste (pastes whatever you last cut or copied to your pasteboard)
Ctrl + Shift + V = Paste in Place (pastes it where ever it was copied or cut from in the same stage place)
Ctrl + Z = Undo last action
Ctrl + Y = Redo last Undone action
Ctrl + A = Select All
Ctrl + Shift + A = Remove all selections

Shift + , and . (this might be a feature in a plugin I use, not sure for certain)
Skip to Beginning or End of Timeline.

Shift + Click
Fill Select between two selections
(extremely handy for selecting a specific chunk of frames without having to scrub your cursor over the entire timeline)

Alt + Drag and Drop -(With Frames already selected)
Duplicate selected frames

COURTESY OF HITSU

Horizontal Alignment
Ctrl+Shift+1
Align things at the left side of the dimension.

Ctrl+Shift+2
Align things at the middle side of the dimension.

Ctrl+Shift+3
Align Things at the right side of the dimension.

Vertical Alignment
Ctrl+Shift+4
Align Things to the upper side of the dimension.

Ctrl+Shift+5
Align Things to the middle side of the dimension.

Ctrl+Shift+6
Align Things to the lower side of the dimension.