Note: You don't need a Home Sharing account to stream content from an iOS device or from a Mac via AirPlay Mirroring; you need Home Sharing enabled only to stream content from your iTunes library. Anyone with a compatible device can stream to the Apple TV, a great party game if your friends are Apple users. (You can also set up a password for your Apple TV in its Settings app to prevent random streaming by visitors.)
You can share photos from computers and iOS devices via the Apple TV to your TV and to other iOS devices and Macs. The Photo Stream feature in iOS's Photos app and in OS X's iPhoto and Aperture apps makes it ridiculously easy to share photos with friends and family. If they have iCloud accounts, they can even comment on them -- great for remodeling projects, family vacation planning, and so on.
Using AirPlay mirroring. If an app or site doesn't have an AirPlay button, double-press the iOS device's Home button, swipe to the left in the multitasking dock that appears, then tap the AirPlay button when you see it to choose a playback destination and enable screen mirroring. Now, nearly anything you watch on your iOS device can be streamed to your TV. I say "nearly" because some iOS apps, such as NBC and Amazon Instant Video, block video streaming via mirroring as part of their DRM.
Note: Some iOS devices don't support AirPlay screen mirroring: the original iPad, the iPhone 4 and earlier smartphone models, and the fourth-generation (late 2011) and earlier models of the iPod Touch.
If you own a 2011 or later Mac running OS X Mountain Lion, you can also stream your Mac's screen to the Apple TV. That AirPlay Mirroring feature lets you stream content from sources other than iTunes to the Apple TV. For example, you can use it to watch videos from Amazon Instant Video, which Apple TV does not support.
Using iCloud. You can also stream your photos, podcasts, and music via Apple's iCloud service. Be sure to enable iCloud on your iOS devices in the Settings app, on your Mac in the iCloud system preference, and on your PC in the iCloud control panel (you can get it from www.icloud.com/icloudcontrolpanel). In the list of iCloud options, be sure Photo Stream is enabled.
On the Apple TV, if your Apple ID/iTunes account is the same as the one you use for your iCloud account, you can access your Photo Stream (including those shared with you by others), your iTunes-purchased music, and your iTunes-subscribed podcasts over the Internet directly from Apple's servers. Apple calls this feature iTunes in the Cloud, and it basically lets you stream what you own directly to an Apple TV rather than go through your iTunes library on your computer or iOS device. If you subscribe to Apple's $25-per-year iTunes Match service, music you purchase elsewhere is also available this way. But videos and other media you got outside of iTunes aren't supported by iTunes in the Cloud.