Android 4.4 KitKat is not yet officially available for Nexus devices, but AOSP builds for the Nexus 4 and Nexus 7 (both models) are available for download.
Owners of the Wi-Fi Nexus 7 (2013) version can get their AOSP KitKat fix from Rootzwiki, while the Wi-Fi Nexus 7 (2012) model has its ROM ready via Paranoid Android. A Google Apps package for them can be grabbed from here.
We will remind you that we don’t encourage you to install unofficial software on your devices, even if these are AOSP ROMs for Nexus devices. You’ll be the only person responsible for whatever happens during the installation, but in case you decide to go this route, make sure you backup your data and follow the necessary install steps closely.
The Nexus 4, Nexus 7 (both models), Nexus 10 and Google Play editions of the Galaxy S4 and HTC One will be updated to Android 4.4 KitKat in the coming weeks, but we’ll certainly keep you updated on the process.