The Android M Developer Preview was just announced at Google I/O during the keynote this morning. Android "M" will focus on "quality end-to-end," with a strong emphasis on improving the user experience, according to Dave Burke.
Supported devices include the Nexus 5, Nexus 6, Nexus 9, and Nexus Player. M includes brand-new features like granular app permissions (which is huge on its own), new app linking APIs, doze mode for enhanced battery life, enhanced fingerprint support (including Android Pay), and a whole lot more.
Google has now detailed some of the ins and outs of the preview, so we've got more information. First, Google claims it will provide developers with clearer timelines for testing and feedback, as well as update preview builds more often. That last one is a biggie. The post seems to imply these previews should be available immediately, but there are no download links as of yet. We'll let you know when we find them.
When the M preview site goes live (here - it's still showing the old L site at the moment), you'll be able to get images from there.