This is pretty powerful online stereogram creation tool, which is actually simple to work with.
Here is an example of stereogram made in Stereogram Paint - [link]
Draw your hidden image, changing levels of Depth and brush shape, then click [Render] button and your stereogram
will be ready in a couple of seconds.
The interface is pretty simple:
1. Background drop down list allows to pick one of preset backgrounds.
2. Texture drop down allows to pick a pattern
3. Brush Type (Circle, Square, Dimond, RectX and RectY) changes brush shape and the slider below changes Brush size.
4. Edge Type (Plane, Sphere, Inset) changes the edge of the Brush and the slider below changes Edge size.
5. Depth slider changes current Z level of the brush
6. Eraser checkbox switches brush to erase mode (don't forget to pick a Depth)
7. Depth Increment is a cool tool. It changes Z Depth of the Brush while you draw. +/- slider sets the speed of change (angle in 3D). If you need to reset it - click the button next to it - 
8. Depth Pick button - [?] gets the depth from where you click - very convenient
9. Unde/Redo buttons - 10 steps of Undo (a must feature in any editor)
10. Clear - erases whole scene
11. Render - generates Stereogram. When you click on stereogram - you'll be switched back to editor mode.
12. Save button - download your Depth Map or Stereogram from
Critics, questions and suggestions are appreciated.
Have fun and don't forget to post what you created in your dA gallery. Give me a link to your stereogram
in comments and I will check it out and will probably leave a feedback
I used rendering algorithm developed by William Andrew Steer ( techmind.org
You might know there another online tool with a similar idea, made by a different programmer - www.flash-gear.com/stereo/
Here is the link, should work in any browser:www.hidden-3d.com/games_stereogram_paint.php