That is, if you have a (most likely lower-end) Windows Phone device that supports it, you can download and run the Windows 10 Technical Preview — for phones. Solely available as a desktop version (previously), Microsoft is ramping up production on the Windows platform, which will span the Xbox, phones, tablets, desktops and laptops.
Besides being unfinished and lacking the new Spartan browser, Windows 10 for phones in its current form does sport some things of note. A better photo browser, universal voice dictation, the new Action Center, and tons of user interface changes.
So! If you have a Lumia 630, 730 or 830 (which I just reviewed), then you can take a look.
Otherwise, just keep your sensors on Microsoft’s development cycle for a while longer.