measuring tool / scale bar

Create a new document. 5000-6000 px is a good size. Size really doesn't matter at this point, as you will be scaling the final ruler.

Draw a shape line using either the rectangle tool or the line tool. for the markers on your scale:

Duplicate the marker and move the new layer to the other end of the canvas, then duplicate that layer for how many divisions + 1 that you want. So if you want a 1 mm scale with 10 divisions, make 11 markers total.

Select all the marker layers and with the move tool selected, click on the distribute centers icon in the menu bar.

Select all the layers where you want the major divisions. In this case it's the two end markers. Press ctrl/cmd-T and scale those markers taller:

Add whatever text you want:

Delete the background layer and then select all the layers and press ctrl/cmd-E. The will merge everything into one vector layer that can be scaled to whatever you want. Save this file for future use. To use it. open you image file and your scale file. Drag the scale file onto your image file. You can then scale the scale to whatever size you need. For this, you don't want to use a smart object, as this is a vector image, and works better outside a smart object.

BarbBinder, yes, it still has that scale, but not a scale where you can add subdivisions at angles to your subject. It's been a while since I've used anything before CS5, so I don't remember if that was an included feature.

If you have the scale bar in the image, you can create a shape layer or several shape layers of different colors and group them. Draw a shape layer the size of the 1mm scale, then transform it and plug in the size you want, say .75 etc. Then you can move and place the marker where you want.