A recent build of Windows 10 has leaked onto the internet today, which includes Spartan, however it's missing a number of important features.
WinBeta has thrown together a quick video demonstrating Spartan in this build, so check that out below. As per usual, we do not recommend installing leaked builds as they are not intended for the public.
Other than Spartan, this build doesn't include anything too interesting that we haven't seen before. Also keep in mind that Spartan in build 10014 is missing a number of core features which make Spartan 'complete' such as Inking, Developer Tools and Extentions.
Tom Hounsell mentions via twitter that this leak has been modified in some way, so those who wish to install this build should tread cautiously. It appears the WIM which leaked has been modified, so keep that in mind.