Here is a small but handy update to Apple's iTunes store: when you hover over an icon for an app, a new little 'i' icon will now appear in the bottom right corner of the screen. Click on it, and a small preview window will appear with all the regular info about the app. You can see screenshots and, of course, also buy the apps, songs and videos right from these previews.
Previews for Apps, Videos and Albums
For videos, a small play icon now also appears when you hover over their respective icons. Press one of these, and a preview of the song or video will start playing. For albums, the preview windows displays the contents of the album.
As Cult of Mac's Alex Heath notes, this looks quite a bit like the "quick look" feature in Apple's Mac OS X.
Here is what it looks like: