Microsoft's Cortana digital assistant debuted with Windows Phone 8.1 this spring, and it (she?) is now headed to Windows 10. The folks at WinBeta have posted a video of a pre-release Windows 10 build with Cortana enabled. Take a look:
Cortana's Windows 10 incarnation is still pretty bare-bones, but it shows some real promise. As WinBeta notes, the assistant can already do most of the same stuff as the Windows Phone 8.1 version, from Skype calls and app launches to reminders and music playback control.
The feature also gives the search button in the taskbar a reason for existing. In public preview builds of Windows 10, the search button essentially duplicates the functionality of the Start menu's search feature. I suppose Cortana could just as well be rolled into the Start menu, though. Hmm.