I'm no artist, but since the iPad announcement, I have gotten excited by the creative image-manipulation possibilities opened by touch-on-a-tablet. What a cool portable canvas!
I addition to mobile apps, some very impressive browser-based applications have come online, so to speak. All of these options give us the chance to express ideas digitally without necessarily having a computer around, or an expensive program whose myriad features we might barely tap.
So I just wanted to name some of the good ones I know and see if anyone wants to add to the list:
Brushes has been on the iPhone for a little while, and was made famous by the artist who created a cover for the New Yorker on his phone.
Sketchpad – Online Paint/Drawing application: My browser couldn't do this before. And it ain't Flash.
Aviary.com: Just heard about this on This Week in Google. Aviary used to cost, but they just slashed the price clean off. Image editing, video effects editing, vector drawing, image markup, sound editing… None of the individual components of this incredible suite of tools would, by themselves, replace their desktop-installed competitors. They're kind of sluggish, and lack ergonomics like shortcuts. But it's a boon to have them available whenever, wherever. And they have a plug-in for Google Apps.
SketchBook Mobile [iTunes] by Autodesk: Looks like the best sketching tool for the iPhone. I like the layers feature a lot. Autodesk is the developer most entrenched among architects, and SketchBook comes in way handy for marking up drawings in the field.
There are tons of photo-manipulation apps for the iPhone, like Photoshop Mobile and TiltShift Generator (both of which, by the way, also have web apps here and here), but I'll just leave off here or I'll be hunting and testing all night.