It's no secret that there's a new Cortana app out there, which separates it from the operating system, allowing it to be updated independently and more frequently than the semiannual Windows 10 update cadence. It's been hidden in the latest Fast ring builds for a few weeks, and it even showed up in the Microsoft Store.
But while you could download it from the Microsoft Store, it only showed a message that said, "Hi, thanks for taking an interest!" As reported by Aggiornamenti Lumia today though, the app finally works for Windows Insiders in the Skip Ahead subset of the Fast ring.
The app does what you'd expect; after all, it's a replacement for the existing Cortana. The big difference in the user experience is that it has its own window, rather than being attached to the taskbar. Naturally, it should be getting more features over time, while the one built into Windows 10 will likely be deprecated at some point.
Note that the fact that it's in the Microsoft Store does not mean that Cortana will be unbundled from the OS. It will still ship with Windows 10, and you probably still won't be able to uninstall it. But now, it can be updated separately.