Selection

Left Click Select item.
Space Add selected item to multi selection.
Escape Clear multi selection.
Delete Delete selected item.

Transformation

Shift + Left Drag Translate
Ctrl + Left Drag Rotate
Shift + Ctrl + Left Drag Scale
Alt + Transformation Snap item to grid while transforming.

Camera

Right Drag Pan Camera
Ctrl + Right Drag Zoom Camera

Creating New Items

Ctrl + Space Clone currently selected entity element.
Alt + E Create Entity
Alt + N Create Script Node (Choose type from type selector)
Alt + D Create Dynamic (Choose type from type selector)
Alt + P Create Polygon - Must have Entity or Entity Element selected.
Alt + H Create Chunk - Must have Entity or Entity Element selected; Must have texture selected.
Alt + C Create Component - Must have Entity or Entity Element selected. (Choose type from type selector)
Alt + I Create Circle - Must have Entity or Entity Element selected.
Alt + T Create Tile Grid - Must have Entity or Entity Element selected.
Alt + G Create Gear - Must have Entity or Entity Element selected. A gear is just a gear-shaped polygon.
Alt + S Create Spline - Must have Entity or Entity Element selected.
Alt + L Create Line - Must have Entity or Entity Element selected.

If a polygon or a polygon vertex is selected...

Alt + Left Click Insert new vertex

If a script node is selected...

Alt + Left click on a terminal, and drag Connect terminals.