Note: LMB, MMB and RMB are the short for Left, Middle and Right Mouse Button, respectively.
Editor and File
Ctrl + S – Save the map.
Ctrl + Z – Undo.
Ctrl + Y – Redo.
Alt + F4 – Close Editor.
Esc – Close Editor (Only if not obtruded by, for example, a dialogue box).
Tools and Navigation
1 through 0 – Select Editor tools (the first 10 tools from top to bottom).
Ctrl+1 or 2 – Select Editor tools (The last two tools in the toolbar).
Alt + LMB – Rotate view.
Alt + MMB – Move center point (or pan view, as you prefer to put it).
Alt + RMB (Zoom).
Mouse Scroll wheel- Zoom in/out.
Q, W – Rotates an object along the Y axis, while placing them in the map.
Q, W, E – Selects the transformation gizmos (Translation, Rotation and Scale, respectively), when the Select Tool is active.
Shift + LMB – Add to/subtract from selection. (When clicking on, or box selecting objects).
Ctrl + LMB – Subtract from selection (When clicking on, or box selecting selected objects).
Space – Expand/Contract active viewport.
F – Focus viewport on selection (zoomed in).
G – Show grid.
Ctrl + F – Find Objects.
Ctrl + D – Duplicates the selected object (In the exact same position as the original).
D – Alternatively, holding D and dragging an object with the translation gizmo will create a duplicate object.
B – Group objects.
Delete/Backspace – Delete selected Object(s).