You can change brush properties while the mouse button is down, by the way.
All it needs now is a color palette. The equations behind this thing are incredible. I was aiming to match Photoshop's brushes and make a Flash program that paints smoother and is more versatile than all the other internet painters. I'd like to use this to make a replacement for iScribble since they use censorship.

Full-screen painter: [link]
.fla file: [link]

The programming
When you click, there's a paint blot; when you drag, more blots. Most programs put a new blot on every pixel you move over, but then the blots overlap too fast and the stroke becomes too dark and solid, but mine follows Photoshop's example and keeps the blots evenly spaced a few pixels apart, giving a smooth stroke. The reason most programs don't do this is because it's hard as shit to code. With each mouse movement, it calculates the mouse's distance from the last blot using trigonometry (xDistance squared plus yDistance squared, then square-root the result for the exact distance) then puts down a new blot if the mouse is the right distance: Half the brush-radius.

This gets complicated when you make a fast stroke, because the mouse starts skipping pixels and you need multiple evenly-spaced blots between the current and previous mouse positions. So the program checks every pixel between the current and last mouse positions, and puts a blot on each correctly-distanced pixel.

The paint blot is a hidden bitmap that's defined at the start of the program, then when you click, it's copied to the visible bitmap (the screen). The blot bitmap is sized 2x the brush radius, and each pixel in it is scanned for its distance from the center pixel. For an aliased (blocky) look, pixels smaller than or equal to the radius would be colored while others are left transparent. A lot of oekaki programs and old MSPaint use this method, but a smooth anti-aliased blot is tricky. For anti-aliasing, you give each pixel an opacity value based on how much of the circle fills the pixel. If, for example, the circle radius is 4, and a pixel is a distance of 4, the pixel will get an opacity of 50% because the circle is only filling half of the pixel (hitting the middle). The pixel's distance ranges from 3.5 to 4.5, so if the radius were 3.7, the pixel would get a 30% opacity because 3.7 is .3 more than 3.5.