Custom rebate router jig
To describe this as a jig might be overselling it, but it shows a very simple way to machine a rebate or dado of arbitrary width to exactly match the thickness of something else. Typically used for cutting dados or rebates to house the ends of a shelf, or for cutting housing or half lap joints. For a more sophisticated variation on this jig see the Routerboard.
You will need
A top bearing fluted cutter for your router, and a couple of bits of scrap stock with at least one straight edge on each, and some clamps.
Start by marking where you want the top or bottom edge of your rebate:
Now offer up whatever it is that dictates the width of your rebate - like a shelf in this example:
Place a couple of spare pieces of wood either side and slide them up so they are snug against the shelf. Clamp in position. (if you are planning to add paint to the shelf and want a little extra clearance then add a sheet of paper or two between the guide and the shelf):
Finally remove the shelf, and offer up the router fitted with a fluted cutter with top bearing.
Plunge it so that the bearing makes contact with edges of the guide pieces, and route away. The guides will constrain the router to the exact thickness of the shelf.
If you need to repeat the rebate in a different position (say multiple rebates for a set of shelves), then you can turn the arrangement into a temporary fixed jig. Screw a couple of bits of scrap wood across the underside of the router guides that mark the edge of the cut - slide the scraps up to the edge of the wood you are routing. This will then hold the position of the guides at the right spacing for the shelves, and also give you a fence that you can use to position the jig squarely. Once positioned, clamp in place and route again. (note that you may need to route into the scraps of wood you are using as your fence if routing the full width of your main board).