Ideas from Adobe is perhaps the best little tool for doodling and a great companion for Adobe Illustrator and Photoshop. You can get it for free and easily add layers for a small fee.
•Now a single app for the iPad and iPhone
•Simple vector-based drawing tools
•Zoom without jaggies or big pixels
•Variable-size brushes using multitouch control
•Vector eraser
• Gallery-style organizer to quickly scroll through your ideas and color themes
•Huge virtual canvas
•More accessible UI: Size, Opacity and Color all available on the main toolbar.
• Automatic creation of harmonized color themes from your photos or images
• Pick specific colors from photos or images easily.
• Separate drawing and photo layers
• Easy creation of multiple versions of design concepts
• 50-level undo/ redo
• Sketches save much faster, avoiding loss of data when you close the app or you need to answer a phone call.
• Save drawing to “saved photos” album on iPad and iPhone (no longer a need to create a screenshot)
• Ability to email ideas as PDF files for editing in Adobe Illustrator and Photoshop or for viewing with any PDF viewer