Gigantt - When you estimate a task using free text you see a live preview of how the system is interpreting it.
/via Assaf Lavie
iOS 6 - When selecting new wallpaper, preview screenshots show a realtime view of your current home & lock screens
/via Rich Cornish
Google search - When hovering over a suggestion, you see preview of the image search results for that term.
/via Floris Jansen
Pixelmator - The “Open recent picture” on the start screen of Pixelmators displays the actual last opened picture right in the icon (see the image in between the brush and the wooden board).
Reeder app - When reading an article, scrolling up and down shows a preview of the previous/next article, and by releasing this article is loaded.