Google released the Android 4.4 KitKat OS along with the Nexus 5 on November 1st and still the official version is yet to be released for other devices.
The new built of the Android is still not available officially for the Nexus devices, but AOSP builds for the LG Nexus 4 and the Nexus 7 are now available for download.
Users of the Nexus 7 2013 Wi-Fi model can get their AOSP KitKat fix from here, while users of the previous Nexus 7 (2012) Wi-Fi model can get their ROM ready, which has been developed by Paranoid Android. A Google Apps (GApps) package for them is available here for download.
As a statutory warning, we will remind you that we do not encourage you to install unofficial software on your devices, even if these are AOSP ROMs for the Nexus devices.
You will be the only individual responsible for whatever happens to your device during installation, but still in case you opt for this route, make sure to back up your data and read the installation steps carefully before starting the process.
The Nexus 4, Nexus 7 (both versions), Nexus 10 and Google editions of the Galaxy S4 and HTC one, all will receive OTA update of Android 4.4 KitKat in the coming weeks. Disappointingly, Google has dropped support for the Samsung Galaxy Nexus.