A special class of Windows Phone 7 apps support live tiles, which dynamically displays information on the start screen. Most of the default apps on the start screen, such as the People hub and the e-mail tiles, are live tiles, but there is a growing number of third party apps in the Windows Phone Marketplace that support live tiles. In the screen shot you see one of my favorite live tile apps, Weatherbug.
Unfortunately, the Marketplace does not provide an easy way to find apps that support live tiles. WMPoweruser.com has created a page that lists a bunch of live tile apps for things like weather, messaging, pictures, sports, and more. After browsing through the list I added the Astronomy Tile Of The Day, which displays a different astronomy picture each day, and TileR 3, which displays pictures from my Flickr photo stream, to my HD 7.