Following the release of full factory images for all supported Nexus devices, Google has begun pushing Android 4.4.3 to the Android Open Source Project (AOSP). The push is ongoing and should be complete within an hour or two.
The build numbers are KTU84M for the Nexus 5 and KTU84L for the rest of the Nexus devices. The AOSP branch is kitkat-mr2-release, with the tag most likely named android-4.4.3_r1 expected to arrive shortly.
Here are several links pointing to the newly released code to get you started:
Update #1: The tag, indeed named android-4.4.3_r1 (KTU84L), has just popped up as well:
Update #2: Another tag android-4.4.3_r1.1 (KTU84M) just appeared:
Of course, you might be wondering what the source code availability means to you. It means custom ROMs, such as CyanogenMod, AOKP, ParanoidAndroid, OmniROM, etc. can start merging the changes and attempting to build Android 4.4.3, maybe even as soon as tonight.