As promised, Google has released the Android L factory images for both the Nexus 5 and Nexus 7, allowing those with either device to install a preview of the latest version of the OS.
To install the factory images, you'll need ADB and Fastboot properly installed, and the process will wipe your Nexus 5 or Nexus 7 clean. If you're really hankering for some Android L but don't want to lose all of your precious data, you might need to sit back and wait for a while. Hopefully, a few custom ROMs with the preview base will become available for other devices as well.
Another thing to keep in mind is that these images are not final, hence the 'preview' status. Neither are likely built for daily use and could have quite a few bugs. So, if you're just looking to check out the new Android L build, make sure you backup your phone first.
If you're ready to flash, head over to the Android Developer site.