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.
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).