For lukep:

In face select mode, select faces by triangle/quad/other.
Available through header menu, toolbox, or hot keys.

Hot keys are:

ctrl-shift-alt-3: Select all triangles
ctrl-shift-alt-4: Select all quads
ctrl-shift-alt-5: Select all non-triangles/quads
(maybe a bit sore for the wrist)

And also available through the menu/toolbox.

I've never used f-gons so I don't know if the 'select other'
function works there (or works at all, for that matter).

source/blender/include/BIF_editmesh.h

@@ -127,6 +127,7 @@ extern void editmesh_align_view_to_selected(struct View3D *v3d, int axis);
 
 	/* Selection */
 extern void select_non_manifold(void);
+extern void select_faces_by_numverts(int numverts);
 extern void select_more(void);
 extern void select_less(void);
 extern void selectrandom_mesh(void);