PhotoToMesh
PhotoToMesh is a standalone Windows application which creates 3D meshes from photographs and other images.  It's perfect for creating your own lithophanes.

The meshes can be saved as DXF files or StereoLithographic files (STL) which you can use for 3D printing. You can rotate the 3D object by dragging the mouse over the screen to see exactly how your object will turn out.

A$75.00

Tinkercad is a great place to start out with Computer Aided Design (CAD) modelling for 3D printing.  

It's actually built for 3D printing, which means that the files that you get out at the end of the process are going to work properly in your 3D printer.  The other totally cool thing about Tinkercad is that it is entirely browser-based, so you don't have to mess about installing things to get started.  All you need to do is to make an account and you are good to go.  (Best also to use a keyword database such as KeePass to keep all of your passwords in :)

It's not the most powerful or flexible CAD software around, but there is a lot you can do with it and it's really easy to use.  It's a bit like SketchUp in its ease-of-use, although it works in a different way.  You have to move the "workplane" around in order to get things positioned where you want them.  Once you get your head around that idea then it all starts to make lots of sense.  Have a look at the fun tutorials on the site, which will have you popping out 3D designs in no time.

Free!
We teach students the basics of 3D modelling using Google SketchUp.
Click here to download SketchUp, for Windows XP/Vista/7 or Mac OS X.

Here is a free SketchUp plugin for that enables you export STL files, required for 3D printing.
The plugin need to be dropped into SketchUp's plugin folder to install it.
  • For PC, that will be in the SketchUp folder within the Program Files folder
  • For Mac, it will be in the SketchUp folder within the Library folder.  Apple have made this slightly difficult by sometimes hiding the Library folder.  Option-click the Go menu.  In Lion, as in Snow Leopard, the user-level Library folder is not included in the Finder’s Go menu. But in Lion, if you Option-click the Go menu, your Library folder magically appears in the menu; choose Library from the menu to open the folder in the Finder. (http://www.macworld.com/article/1161156/view_library_folder_in_lion.html)

There are other useful plugins for SketchUp as well, particularly:
Pepakura Designer allows you to easily create papercraft models from 3D data you create from your favorite 3D software.