Apple has updated its free Apple Remote app for iOS devices, adding in AirPlay video support. The app [iTunes link] now allows you to choose from video media stored on your computer and control its streaming to an Apple TV. Meanwhile, there's also control over internet radio playback, along with access to rented TV shows and movies on iTunes on your desktop.
As before, you can also use the app to create and edit playlists (including Genius playlists), send audio via AirPlay to speakers, and to generally operate as a more comprehensive remote control for the Apple TV. You'll need an iPhone, iPod touch or iPad to use it; you can download it here.