Xiaomi devices don't have a big footprint outside of Asia, but there are a ton of these phones in the world, and they all ship with the MIUI build of Android. Now, the final build of the new MIUI 7 is set to roll out on October 27th via OTA on supported devices. This version of Android brings a ton of improvements, but it's weirdly still based on KitKat for some devices and Lollipop on others.
MIUI 7 has been in open beta for about a month, so the feature set is well known. The new software extends battery life by 10% and speeds up system performance by 30%, or so Xiaomi claims. There are also included system UI themes, as well as a theme builder and a ton of downloadable themes. Then you get some smaller tweaks like incoming call videos for your contacts, a kids mode, and an enhanced gallery with facial recognition.
MIUI 7 will arrive via OTA on newer Xiaomi devices including the Mi 4i, Mi 3, Redmi 2, Redmi Note Pro, and Redmi 1S. A flashable ROM download will also be available on the MIUI download page for supported devices. There are MIUI 7 builds listed there now, but I think these are still beta builds. Keep in mind, some of these phones are staying on KitKat with MIUI 7, but it's not clear which ones yet.