The folks from Adobe Press have introduced Adobe Photoshop CS4: Learn By Video. The application introduces the most essential topics in Photoshop CS4. Users can:

• Take a quick tour of the Photoshop CS4 interface

• Mark any movie for later viewing

• Share tips and comments with others
• Quizzes: Test yourself! Track your progress and review problem topics.

• Stay up-to-date on Adobe Press news through the Twitter group

The companion video package features 19 hours of training from Gabriel Powell and Mikkel Aaland, as well as quizzes & review materials.
In a similar vein, Richard Harrington’s Understanding Photoshop: Quick Fixes (iTunes link) offers the following:

• Includes 17 training videos edited specifically for the iPhone or iPod Touch.
• Offers easily viewable screens, with zooms and close-ups of the action.
• Includes hands-on files & interactive quizzes.
• Includes search, a quick reference guide, commenting, and a Twitter client.

These are just the apps I’ve encountered so far. If you know of other good ones, please mention them via comments.