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)

Editing

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

Objects

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)