Features:

Auto Entering Serial.
Selecting the KMS server.
KMS activation.
KMS activation (bottom constantly tries to find and servers for activation).
Inspired by the Metro UI.
Installing the Language Pack.
Rearm Windows (works in 8 too).
Auto Update Server.

What's new in v1.1:

New KMS server added.
Fixed problem where could not detect the server chosen.
Fixed issue with popups command.
Improved launcher background.
More bug fixes.

Cautions: 

Windows Defender may detect this file as virus. I think as it is a cracking tool that’s why defender may consider this as a threat. But this is safe (Though I’m not 100% sure but using it on my PC).
Net Framework 4.0 is required to run this activator. If your Windows doesn’t have .Net 4.0 installed then install it now.

What if it does not work:

Make sure it is running as Admin.
The KMS server may be slow, more servers will be included in future releases.
Make sure you have the appropriate KMS key that you need to install, more keys and automatic detection of OS will be added in future versions.
Error saying something about non-core, you are entering a key that is not for your version.
Do not use a proxy.

How to use?

Open Windows 8.1 RTM KMS Permanent Activator v1.1
Choose OS version.
Choose KMS server.
Run activator with a click on “Activate Windows”