Review our UI mockup tool, ScreenSketcher
petervandijck: Here's a thought: make the download free, add an online (simple!) sharing service (to share with teams and annotate/comment, behind a simple password), and charge a small subscription fee for that. Say 9$/month per team (unlimited users), or a graded pricing (9$/m for up to 6 users, 49$/m unlimited team/users) will get you the same price as the app in 7 months or less, and more after that (depending on churn of course). Keep the download app free. Then improve the app a little, but don't add lots of features. (DO add iPhone stencils, and small UI improvements like editing inline instead of in a popup box).