Windows 10 Insider Preview Build 10576 is out in the wild, and with it comes a fairly exciting feature, if you have the device(s) to support it: media casting.
Via Edge, you can send videos, pictures, and audio to any Miracast and DLNA enabled device in your home. This includes support for YouTube videos, Facebook photos, and Pandora songs, but in theory you may be able to cast just about anything. Sadly, protected content is not supported, so you can't cast Netflix, Hulu, etc. -- one of the key reasons to cast in the first place.
Other changes in this build include Xbox beta app voiceover recording and search and purchase functionality, a fix for the Xbox beta RAM usage issue which popped up under specific circumstances, and Hyper-V performance improvements.