"Play To is the ability to right-click a media file in Windows 7's Media Player and send it to any networked gadget appearing in the pop-up menu. It's sweet now, but it could be huge.
The key is that you have to have compatible networked "digital media renderers" at the receiving end. Right now, the list is verrrry short: Sonos is the poster child for the feature at this point, and when you right click a song and send it to the Sonos ZonePlayer in any room, it starts playing no problem. But Sonos is for music only and so is Roku's SoundBridge, which has a variation of the feature enabled on its latest firmware. Video is the holy grail, but for now, the only thing you can send video to is the Xbox 360—and then only when it's running Media Center Extender.
This will change soon, when the DLNA rolls out its "Play To" certification as part of the 1.5 specification. The functionality will likely find its way into loads of media-rendering products: The PS3 is key, but ideally this will be in TVs, connected stereo systems, media adapters and digital picture frames too. Can you imagine how nice it would be if sending a photo to a frame was a right-click away, instead of some convoluted 12-step process requiring proprietary software and a steady easterly breeze?"