Software for iPad: MyPaint (Free)
You don’t expect much from a free painting app, but MyPaint is rather like a box of felt-tip markers – what you can draw with them is really up to your skills and abilities and the software doesn’t help you (or conceal your artistic failures) nearly as much as some of the other tools. The controls are basic and you don’t get the natural painting effects of more complex software; you can set the size of the brush and the transparency of the paint, and the color picker has 35 pre-set colours plus drag bars for adjusting the red, green and blue values rather than the more sophisticated hue, saturation and value options. You can also press and hold to select a color from the painting that you've already used; nice if you like the effect you get where colors overlap.
As for painting, all you can really do is draw and erase (there’s an option to use a photo as a reference image but we found this wasn’t reliable), or zoom in and out to work on different areas. The lines you draw aren't as smooth as we’d like, especially with a thinner brush. But just as markers are fantastic tools in the hands of a talented artist, you can still do some great art with MyPaint if you draw carefully; the gallery has some good examples of how good or bad the results can be. The rest of us might find natural media tools more inspiring – and forgiving.