Shortcut: 5, Ctrl+Alt+V
The PolyPen tool provides absolute control for creating complex topology on a vertex-by-vertex basis (e.g., low-poly game models). This tool lets you insert vertices, extrude edges, fill faces, and transform the subsequent geometry all within one tool and in just a few clicks.

Ctrl+LMB, Ctrl+LMB+Double |
: | insert geometry connected to selected geometry |
LMB+Click, Shift+LMB+Click |
: | select geometry |
LMB+Drag, Shift+LMB+Drag |
: | paint geometry selection |
Ctrl+Shift+LMB+Click |
: | select along shortest path |
A |
: | select / deselect all |
Alt+A |
: | deselect all |
G |
: | grab and move selected geometry |
LMB+Drag |
: | grab and move selected geometry under mouse |
Shift+S |
: | smooths edge flow of selected geometry |
R |
: | rotate selected geometry |
S |
: | scale selected geometry |
Backspace, Delete, X |
: | delete/dissolve/collapse selected |
V |
: | rip selected edge |
Alt+V |
: | rip and fill selected edge |
Creating vertices/edges/faces is dependent on the selected mode and the selected geometry.
Switch between modes using the PolyPen Options, or hold Shift+Q, ~ to bring up a the PolyPen pie menu.

The Tri/Quad mode is the normal mode for PolyPen.
Selecting an edge and clicking onto another edge will create a quad in one step.
The Quad-Only mode reduces the number of clicks for creating a strip of quads.
Selecting an edge and clicking onto another edge will create a quad in one step.
The Tri-Only mode is similar to the Tri/Quad mode, except without the step that turns the selected triangle into a quad.
The Edge-Only mode inserts only edges.
The PolyPen tool can be used like a knife, cutting vertices into existing edges for creating new topology routes.