I'm a software developer and am working on a personal project (website + iphone/ipad app). I need simple illustrations for these to fill in the blanks until the project gets "some" paying customers or I find investors. With more money I can hire some graphic design folks to do a great job. As for now, I'm aware of the big brand tools out there (Photoshop, Illustrator) but am looking for an illustration tool for beginners. I've used "Paper" app by FiftyThree Inc. and was able to create something fairly decent in just a few minutes. I'm looking for an application that will do this on my computer, that has more features than Paper, but without the learning curve of Photoshop or Illustrator.
Summary question: What are some great beginner illustration tools (OS agnostic)?