I have need to mount a servo onto a shelf facing sideways.

This thing will allow you to put a servo into a snugly fitting sleeve, and then slide that sleeve onto a shelf.

The OpenScad will let you play with the sizes, including an offset which is used to specify the height of the mount (0 would be flush with the shelf).

I took my measurements from this servo from AdaFruit: http://www.adafruit.com/index.php?main_page=product_info&cPath=34&products_id=155

I think it's a fairly standard size.

Note that you typically have to remove the top or bottom from the servo to fit it into the mount. Taking off the top is easiest.

1) Look at your robot design for places where a side mounted servo might be useful.
2) Print one of these for each servo
3) Remove the top of the servo so you can slide the servo into the "back" of the sleeve.
4) Once naked servo top is flush with the sleeve opening, replace the lid, and screw it down.
5) Press the servo flush with the sleeve until the mounting holes are resting on the mounting hole tabs of the sleeve.
6) Slip the assembly onto a shelf
7) Rejoice!

This thing has a very tight fit. If you want a slighly looser fit, alter the OpenScad file, in particular the Depth and/or Width values.

If the friction fit is not enough for your needs, then use some screws and nuts to secure the mounting holes on the servo to the mounting hole tabs on the shelf clip.