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via inhabitat: 5 most useful apps for artists and designers
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Brushes
Brushes has a simple yet powerful UI (unlike SketchBook Pro), and is often considered the best app for digital painting. This app's toolbar is located at the bottom of the screen and includes a color picker, eye dropper tool, paint bucket, brushes, brush properties and an eraser. On the bottom right of the screen, users can redo or undo any action and have direct access to all of their layers. Because this app was developed specificly for painters, the advanced and accurate brush features and capabilities are impressive but not surprising. It is awesome.
$4.99 at iTunes

Palettes make it easy for anybody to create a color palette anywhere at anytime. You can grab colors from a photograph, website, or any one of 5 color models offered. And if you are really serious about color, definitely consider paying the extra dough for the Pro version.
One of the best features of the pro version is that it allows you to Import and Export palettes to and from Adobe Photoshop, Illustrator, and Fireworks, GIMP, Paintshop Pro and OS X Color Chooser. This app is a must have for graphic and web designers.
$5.99 at iTunes

Rulers 2
We all know the importance of accuracy, and so does Rulers 2, which has given us the tools to be accurate and on point everywhere we go. Available for your iPhone and your iPad, this app will turn your mobile device into a handy measuring tool. Rulers 2 features a pointer with a digital readout, so all you have to do is line up the device with what you are measuring, and poof! a precise measurement is at your finger tips. Literally.
The added beauty of this app is that you actually get a pointer with a digital readout. This means that all you have to do is line up the device with whatever you are trying to measure and you will get the precise measurement instantly. This measurement will be displayed in inches--complete with fractions--or in centimetres with decimals, depending on which option you choose.
$0.99 at iTunes