Steam for Linux has been in the Beta stages for some time now, but Valve has released the final version of this branch.
Even if the final version was made available for the public, the development branch continued to stride on and Valve has finally included all the changes from the Beta into the stable branch. This means that for now, at least, there is only one Steam for Linux version.
The Opt-in for the Beta will still be available for anyone wanting to be on the bleeding edge, but it's not going to provide a different version from the official one.
Highlights of Steam for Linux:
• Sorted download regions in the Settings page for Content & Downloads;
• "Send Message" has been added to the context menu for offline friends;
• The appearance of non-English text in the auto-update window has been repaired;
• A --reset command line option to reinstall Steam files without touching installed games has been added.
Check the official announcement for more details about this important release.
Download Steam for Linux installer 188.8.131.52 right now from Softpedia.