Another useful tool for efficient usage of WeightPaint:

Press SHIFT+LMB, and it shows a menu with all Vertex Groups the vertices of
the indicated face has, also allowing selection of Groups.

source/blender/src/vpaint.c

@@ -813,12 +813,14 @@ static MDeformWeight *get_defweight(MDeformVert *dv, int defgroup)
 	return NULL;
 }
 
-/* used for 3d view */
+/* used for 3d view, on active object, assumes me->dvert exists */
-/* cant sample frontbuf, weight colors are interpolated too unpredictable */
+/* if mode==1: */
-/* so we return the closest value to vertex, wich is actually correct anyway */
+/*     samples cursor location, and gives menu with vertex groups to activate */
-void sample_wpaint()
+/* else */
+/*     sets editbutvweight to the closest weight value to vertex */
+/*     note: we cant sample frontbuf, weight colors are interpolated too unpredictable */
+static void sample_wpaint(int mode)
 {
-	extern float editbutvweight;
 	Object *ob= OBACT;
 	Mesh *me= get_mesh(ob);
 	int index;
@@ -829,15 +831,82 @@ void sample_wpaint()
 	
 	if(index && index<=me->totface) {
 		MFace *mface;
+		
+		mface= ((MFace *)me->mface) + index-1;
+		
+		if(mode==1) {	/* sampe which groups are in here */
+			MDeformVert *dv;
+			int a, totgroup;
+			
+			totgroup= BLI_countlist(&ob->defbase);
+			if(totgroup) {
+				int totmenu=0;
+				int *groups=MEM_callocN(totgroup*sizeof(int), "groups");
+				
+				dv= me->dvert+mface->v1;
+				for(a=0; a<dv->totweight; a++) {
+					if (dv->dw[a].def_nr<totgroup)
+						groups[dv->dw[a].def_nr]= 1;
+				}
+				dv= me->dvert+mface->v2;
+				for(a=0; a<dv->totweight; a++) {
+					if (dv->dw[a].def_nr<totgroup)
+						groups[dv->dw[a].def_nr]= 1;
+				}
+				dv= me->dvert+mface->v3;
+				for(a=0; a<dv->totweight; a++) {
+					if (dv->dw[a].def_nr<totgroup)
+						groups[dv->dw[a].def_nr]= 1;
+				}
+				if(mface->v4) {
+					dv= me->dvert+mface->v4;
+					for(a=0; a<dv->totweight; a++) {
+						if (dv->dw[a].def_nr<totgroup)
+							groups[dv->dw[a].def_nr]= 1;
+					}
+				}
+				for(a=0; a<totgroup; a++)
+					if(groups[a]) totmenu++;
+				
+				if(totmenu==0) {
+					notice("No Vertex Group Selected");
+				}
+				else {
+					bDeformGroup *dg;
+					short val;
+					char item[40], *str= MEM_mallocN(40*totmenu+40, "menu");
+					
+					strcpy(str, "Vertex Groups %t");
+					for(a=0, dg=ob->defbase.first; dg && a<totgroup; a++, dg= dg->next) {
+						if(groups[a]) {
+							sprintf(item, "|%s %%x%d", dg->name, a);
+							strcat(str, item);
+						}
+					}
+					
+					val= pupmenu(str);
+					if(val>=0) {
+						ob->actdef= val+1;
+						DAG_object_flush_update(G.scene, ob, OB_RECALC_DATA);
+						allqueue(REDRAWVIEW3D, 0);
+						allqueue(REDRAWOOPS, 0);
+						allqueue(REDRAWBUTSEDIT, 0);
+					}
+					MEM_freeN(str);
+				}
+				MEM_freeN(groups);
+			}
+			else notice("No Vertex Groups in Object");
+		}
+		else {
 			DerivedMesh *dm;
 			MDeformWeight *dw;
+			extern float editbutvweight;
 			float w1, w2, w3, w4, co[3], fac;
 			int needsFree;
 			
 			dm = mesh_get_derived_deform(ob, &needsFree);
 			
-		mface= ((MFace *)me->mface) + index-1;
-		
 			/* calc 3 or 4 corner weights */
 			dm->getVertCo(dm, mface->v1, co);
 			project_short_noclip(co, sco);
@@ -877,9 +946,9 @@ void sample_wpaint()
 					if(dw) editbutvweight= dw->weight; else editbutvweight= 0.0f;
 				}
 			}
-		
 			if (needsFree)
 				dm->release(dm);
+		}		
 		
 	}
 	allqueue(REDRAWBUTSEDIT, 0);
@@ -936,7 +1005,11 @@ void weight_paint(void)
 		create_dverts(me);
 	
 	if(G.qual & LR_CTRLKEY) {
-		sample_wpaint();
+		sample_wpaint(0);
+		return;
+	}
+	if(G.qual & LR_SHIFTKEY) {
+		sample_wpaint(1);
 		return;
 	}
 	
@@ -1172,7 +1245,6 @@ void weight_paint(void)
 	if(ob->soft) ob->softflag |= OB_SB_REDO;
 	
 	allqueue(REDRAWVIEW3D, 0);
-
 }
 
 void vertex_paint()