After the official release of the Nexus 5 and its accompanying Android 4.4 operating system, Google promptly released developer benefits, including the factory system images (commonly referred to as "base packages") and various drivers.
The internal code name for the GSM/LTE version of the Nexus 5 is "Hammerhead" (hammer), and the current build number for Android 4.4 is KRT16M. Those interested in researching custom ROM flashing can now begin their work.
The drivers include Qualcomm's GPU, GSM, camera, GPS, sensors, media, DSP, USB; Broadcom's NFC, Bluetooth, Wi-Fi; and LG's camera, sensors, and audio.
Qualcomm had a falling out with Google over open-source drivers in the past, but now they are providing drivers concurrently, which shows a decent attitude that should continue.
As for when the rest of the Nexus family will be upgraded to Android 4.4, there has been no official announcement yet.
Factory image:
https://dl.google.com/dl/android/aosp/hammerhead-krt16m-factory-bd9c39de.tgz
Qualcomm drivers:
https://dl.google.com/dl/android/aosp/qcom-hammerhead-krt16m-53cf1896.tgz
Broadcom drivers:
https://dl.google.com/dl/android/aosp/broadcom-hammerhead-krt16m-bf9b8548.tgz
LG drivers:
https://dl.google.com/dl/android/aosp/lge-hammerhead-krt16m-0efa9c33.tgz