Android L images

This part is important:

The Android preview system image is not a stable release, and may contain errors and defects that can result in damage to your computer systems, devices, and data. The preview Android system image is not subject to the same testing as the factory OS and can cause your phone and installed services and applications to stop working. 82

Have fun. Be careful.

Get the factory images for Nexus 5 and Nexus 7 here.