It's the moment you've been waiting for. Google just announced the Nexus devices are getting OTA updates to 5.0, and now the full system images have been posted on the Google Developers site. With these files, you can flash your device back to stock even after you've done unspeakable things to it.
The images we have so far are for the Nexus 5, 7 (both WiFi models), 9, and 10. There's no Nexus 6 image yet.
Android 5.0 Lollipop Factory Images And Binaries Are Live For Nexus 5, 7 (2012 And 2013), 9, And 10 was written by the awesome team at Android Police.