You don't need a plug in for that, it's been built right into Photoshop for quite sometime.

Once your selection is made go to EDIT/STROKE

OR

Go to LAYER/LAYER STYLE/STROKE

Both of these will give you the option to run a line around your selection. You determine the thickness of the line, the color of the line and whether the line runs around the inside, outside or center of your selection. The first option adds it directly to the image. The second adds it as a layer effect so it's easier to make modifications to it if you want to. Of course the selected area has to be on it's own layer for that to work.

A different approach you can use if the selected are you want to ouline is on it's own layer is to have it selected and then go to SELECT/MODIFY/EXPAND. Place a layer behind the layer with your selection on it and fill it with color. Sometimes this can produce a smoother result than the stroke depending on how thick you want the outline to be.