Google has begun shipping the Google Play editions of the Samsung Galaxy S4 and the HTC One, directly from the Play Store, just like the Nexus devices from Google. By now I guess everyone knows what these Google Play editions are, but just in case, I'll explain it briefly.
They're the exact same hardware that Samsung and HTC manufactured for their flagship devices, but with Google's stock Android software, the way it was conceived by Matias Duarte and his team, same UI and same UX that the Nexus phones, even more advanced in a few areas, like the camera, which is a newer version than in the Nexus 4. Of course that hasn't stopped anyone from getting the new camera UI into any other device.
On top of vanilla Android you get SIM-unlocked devices with LTE support in the US and the latest hardware on the market. Samsung's phone retains the flip cover functionality in the lockscreen and the HTC One still has Beats audio enabled (you'll need it for BoomSound) and some camera code for the Ultrapixel sensor. Other than that, it's stock Android.
Neither of these devices will be updated directly by Google, they just handle Nexus devices which are 100% supported by the AOSP with no proprietary drivers or anything like that. Google Play editions still need some Samsung and HTC code to work properly with the hardware (like HTC's camera) so they will be handled by each company respectively, both in updates and support. However, Google did say they will be working together with Samsung and HTC to get updates in a timely fashion, and since we've already seen a leak of Android 4.3 for the Galaxy S4 and the HTC One just passed through the FCC for Bluetooth LE with Android 4.3, it's safe to assume that even if the update isn't at the exact same date as the Nexus 4, it will be pretty close.
If you like the stock Android experience, you now have 3 choices: Google Nexus 4 (at $349), Samsung Galaxy S4 (at $649) and HTC One (at 599), not a bad lineup if you ask us. Now Google has more ways to get stock Android to people and could start taking some mind share out of some of those pesky manufacturers skins.