From top to bottom:

• The Septagon – This option returns for anyone who links their LIVE account to their Bungie.net account.
• The DMR – A pro weapon for Pro members. Get Bungie Pro to bump up your File Share, render videos, and get this emblem. Active subscriptions bought in Halo 3 or Halo 3: ODST carry over to Halo: Reach.
• Marathon – Play the full version of Marathon: Durandal on Xbox LIVE Arcade to enable the Pfhor Terminal emblem.
• Halo 1 – Enter your Halo 1 PC/Mac product key to enable this emblem. Each key can only be used once, so you’d better stop reading this and grab that key before your brother does. 
• Halo 2 – Automatically available if you played Halo 2 for Xbox on your LIVE account before its tearful retirement. If it’s not active for you, no amount of wheedling will change that, sorry.
• Halo 3 – Take a guess… On the plus side, there’s plenty of time to head to your local purveyor of fine, shrink-wrapped games to snag a copy if you’re the one person who actually reads the news on our site and hasn’t played this game.
• Halo 3: ODST – Ditto.
• Assault Rifle – This fine armament goes to anyone who played the Halo: Reach beta a few months ago.
• Spartan Helmet – CLASSIFIED
• HALO – Like Blamite out of a Needler, these faceplate options can supercombine. Unlock the emblems for Halo 1, Halo 2, and Halo 3 to get this special emblem.
• The Star – Who knows?