Google has just released the Android L developer preview system images for the Nexus 5 by LG and the 2013 WiFi-only Nexus 7 by ASUS (sorry, LTE fans).
Here are the direct links:
These are full system images, which means you flash them just like any other Nexus factory image. For a really great tutorial, check out this XDA post.
Be forewarned: these are unstable, development images that have not gone through the same testing that normal factory images have. While these are, for the most part, stable and usable, it is probably not recommended to use these as a daily driver for your one and only phone.
Here’s a good reason why: I just got done flashing this and setting it up on my Nexus 5, and I thought everything was grand, but then my phone started going through a bootloop out of nowhere. The only way for me to recover was to boot into the bootloader and reflash the factory image. I have no idea what happened – I didn’t try to root anything, and all I really did was install some apps and set up a widget on my homescreen. So obviously, your mileage may vary.
If you decide to give this a spin on your Nexus device, let me know how it works out for you! I’ll be posting some of my observations once I’m done playing with it. As long as I don’t get any more bootloops.