MyPaint lets you draw beautiful freehand pictures
I love freehand painting. While I have absolutely no talent, I love the freedom of working with paint or even crayons. It just feels nice, even if the result is nothing to write home about (in my case, at least).
If you're looking to replicate the same experience on a computer, there's always Corel Painter. But on the off chance that you don't have hundreds of dollars to spend just to doodle, MyPaint gives you a nice chunk of functionality for the even nicer price of... free.
The program comes with many brushes that are divided into several categories. The brushes seem to work quite well at default settings. In case you want to tweak something, though, hitting Ctrl+B shows the Brush Settings dialog, which has a mind-boggling array of parameters that you can change for the brush you've selected.
The interface is multi-windowed but is not as horrible as GIMP's (sorry, GIMP lovers). The color picker is quite advanced and features color-matching harmonies (which I completely failed to use in my creation shown above).
Right-clicking the canvas rotates between the last five colors selected, which is a very handy feature. The program also features a ton of single-stroke hotkeys for changing brush sizes, saving favorite brushes, and more. MyPaint even supports layers, and it is available for both Windows and Linux.
Bottom line: This is not a toy. Whether you are serious about painting software or just starting out MyPaint is a great option.